He prowls into bedrooms of Mother in Law's house

I can't believe that as soon as she heard him walking about upstairs she didn't immediately go up and see what he was up to or at least say to her daughter "Why has 'John' gone up stairs?"
What is the problem? I must be missing something here??

I meant this snooping behaviour is unacceptable not MawBroon's suggestions!

Unacceptable! She should speak to her DD about it. After that it's up to her to sort her husband out. If he does it again don't ask them back again. Creep!

How about the spy trick of the bit of blu-tack on the door frame? The hair on the edge of the drawer?
Or a note on the bed?
Seriously , you have to be able to trust your family so this needs to be cleared up ASAP

Just wondering, would friend have reacted in same way if DD or GC had been in the bedrooms or study? Maybe SiL is just nosy.

Could she not visit them and whilst there tell them calmly that she is uncomfortable with his prowling around her home and ask his reason.

Could he possibly be checking that everything is OK - roof not leaking etc? Although if she is finding things have been moved that seems unlikely.

Some good ideas there Elegran I really don't see why this lady should have to put locks on her doors at all. Sometimes there has to be a confrontation however tricky that may be.
I hope your friend is able to resolve this Tillybelle.

She really should have asked her daughter to go up and check that he was ok, or was he looking for something specific? That way the SIL would know that they were onto him. Unless he does it again when they are all together it is going to be a big deal to try and casually bring it up in conversation, meanwhile what if he turns up on her doorstep unexpectedly ? he will scare her to bits.
She really needs to talk to the family about this and not have to put locks on her doors, poor lady.

If she plans something to say, she won't be struck dumb next time, she'll speak out. If he's not stopped he will keep on doing it.

Perhaps the next time she hears unexpected noises upstairs when they are visiting, she could act she is afraid, and say so, out loud "There must be an intruder upstairs! You go and check!" As the oldest there, she is not going to confront a burglar herself, of course! When SiL is revealed, she could say that there is no-one in the downstairs loo, it is empty so he doesn't need to go upstairs - and add with a laugh that the rooms don't need tidying up there either or she won't be able to find anything.

If he doesn't take the hint she may have to repeat the process, and possibly say that she is grateful for him wanting to help tidying but she'd really rather do it herself.

tinybell it is definately not being unreasonable to be upset. I would be unable to let this happen a second time. No-one should have to put locks on their doors. I would have to ask 'why do you have to go upstairs'? and request him not to. I would also explain to your DD that is not acceptable when there is no reason, OCD or not.

Grannyknot Me too - I suggested along such lines. There are reasons though, things are not easy for her. i am hoping she will be feeling confident enough to ask, in a very unemotional way, smiling politely, "What exactly were you looking for upstairs in my house?" However, I think she may not want to rock a rather delicate boat. I really feel for her. She is a special person, so kind and good to people.

Ugh! How creepy - what on earth is he after, nothing that doesn't fill me with horror - her jewellery to sell on or her underwear to put on! Maybe he just likes looking at peoples houses? Whichever, it's weird and I think her DD should know about it!

I don't understand the "treading on eggshells" bit and not upsetting her daughter (she's not the one snooping) - in my house I'd ask "What are you doing upstairs?" or "What are you looking for?" or "Is there something you need?" especially if it's family.

All perfectly reasonable questions ...

Upset? - so she should be!

I need to add, my dear friend wanted me to post this.