He prowls into bedrooms of Mother in Law's house

If he is on the autism spectrum he'd probably appreciate being told in a straightforward way that it's not on to wander about in someone else's house. Take the emotion out of it and just offer the facts.

Why didn't he take the 'hint' when she put a padlock on her study door?

People on the autistic spectrum don't recognise hints. They like things to be cut and dried with no nuances.

I find it hard to understand how she's afraid to say anything.

There doesn't have to be a row - she could just quietly tell him that anywhere (presumably) but the loo/bathroom is off limits, and will he please not go into other rooms again. And refuse to discuss any further.
The SiL is the one who's being U - not your friend.

No, it wouldn't be very easy or pleasant, but it def. needs to be said.
(My inner nasty cynic can't help wondering whether he's looking for her will!)

I am of the "speak first and think later " nature ! I would not think twice about asking him what he was looking for , surely her DD is aware of what he is doing , no one prowls round my house and even when SIL comes in to do some task for me , he hovers in the doorway and won't come in till invited

I don’t know ladies, my childhood best friend is on the spectrum and I have never in thirty years had to specifically tell him not to go through my stuff or inspect my house when my back was turned.

I think a bloody locked door is something most folk I know on the spectrum means ‘no entry without permission’.

It’s difficult not to impute bad motive to SIL here on the facts as presented. At best, this behaviour is extremely weird.

Tilly Could he be checking to see if your friends partner is maybe not looking to become a more permanent fixture in her life and could upset any inheritance he may have his eye on.
If he has done this before and comes across a locked door this time he goes snooping surely he would realise he has been rumbled if he is so clever,I wouldn't put up with it and tell him straight he needs to use downstairs facilities from now on.

I too wondered about aspergers or similar and maybe there’s a few clues in one of your posts you talk of his pristine and neat home (might be a bit of OCD there) then you mention his ‘geeky’ friends again a bit of a clue??? Of course none of this excuses his behaviour but maybe his guidelines are different to others
I still find it remarkable that nothing was said I can’t imagine looking up and seeing him up my stairs and not either saying ‘ what are you wanting up there’ or at least a bit more humorous with a ‘ have you got yourself lost’ or something similar I cannot see why your friend doesn’t speak to her daughter about it not in an accusation way but just a questioning way
It all sounds really bizarre

Sorry to ask this but is there any possibility of sexual fantasies going on in his mind? We came across a similar situation with a teenager female who had fantasies about her Uncle. In the end we caught her going through his clothing on a family visit and she admitted her fantasties were of a sexual nature.

I still don't understand why he wasn't asked what he was doing the first time he was caught out. Just a spontaneous reaction to seeing this odd behaviour.

Interesting that you mention autistic spectrum as I have a cousin who I suspect to have ASD who does this. Once I went to put something into the oven and came back to find her rooting in my cupboards and drawers.

People with ASD often don't understand normal boundaries - so if, for example, he was interested in looking around the house it might not occur that this is not socially acceptable. My dd and her bf (both diagnosed with Aspergers) don't get social norms either - they'll both wander into my bedroom for a chat even if I'm in bed in my nightie!

Various motives for the son in law prowling around his mil's bedroom have included bizarre sexual fantasies, checking out inheritance possibilities and being on the autism spectrum and not understanding boundaries. Standing at the top of the stairs and staring down at his mil when he was 'sprung' sounds more like very nasty bullying behaviour. And as we know all too well being intelligent in a well paid position does not mean someone is not a nasty bully. Quite the opposite.

Whatever the reason for son in law's prowling the fundamental issue is that it is rude, intrusive and totally unacceptable behaviour as a guest. You do not enter private rooms (which includes bedrooms and studies) in someone else's house without the person's knowledge and permission. You do not need to put locks on doors in your own house but you may need to manage your guests behaviour.

The fact that this needs to be spelt out to the son in law and everyone is worried about upsetting the daughter suggests that there is something very odd about the sil and the relationship between the daughter and her mother. The sil needs to be told very clearly that it is not acceptable behaviour and if it happens again he will not be welcome to visit. Bullies need to be called out.

Totally agree with your comments Apricity. Good post.

Mousetraps in the drawers.

It's unacceptable in my opinion. I wouldn't be able to resist saying something to him about it.

Your friend needs to tell her daughter, as unemotionally as possible, that she does not want her son in law to go into other rooms in her house without invitation, and leave her to deal with it as she will.
When the son in law visits again she needs to watch him like a hawk, follow him if he leaves the room, and again, unemotionally as possible, ask him directly if he is looking for something.
She also needs to check her possessions.
My uncle, many years ago, used to stay with my grandmother for brief visits and never went home without something: towels, travel blanket, old family papers, and his excuse was, when confronted, well, she never uses them, and I am her son. After he died and we had to clear his house, we were shocked at the amount of things he had accumulated, much of it actually belonging to my father. As a bank manager, one would have expected him to be a model of honesty.