Tenant has met a girl......

you have to remember the world has moved on since the 60's .We couldn't get a rental flat without a marriage certificate .Young people nowadays would think thats ridiculous and rightly so.
If you want him to consider it his home then he should be allowed friends ...even girlfriends to stay .Its the norm for most families that their teens have girlfriends to stay over .What is your objection to it?He's your tenant not your child so his lifestyle isn't really your business as long as he stays in the rooms he's allowed to use .

You really should have planned ahead but if you don't want the GF staying over tell him it's not allowed. After all he's only paying rent for one not two. If use of the lounge is not included in the rent remind him of this too, you shouldn't be feeling uncomfortable in your own home. Of course if he wants the GF staying over he may look for new lodgings.

When I went to live in a bed sitter in 1969 in Manchester, my landlady laid out the ground rules before I moved in.
There was only one.
I could have one boyfriend staying over, but not a whole string of them. Her meaning was perfectly clear.
You’ve left it a bit late Denil, but decide what you will tolerate and tell your tenant now.

Has met a girl and bought her back. Spelling edit !