My AP did not have a boyfriend when she came, but she got one fairly quickly.
He would come visit with her the occasional evening (actually, sometimes 3 times during one week, but more like twice a month), she would heat up some left over dinner, make him an omelette or something pretty cheap and easy, and they would withdraw to her room with a cup of tea. He never stayed past midnight. However, I sometimes found it tiresome to come down after putting the kids to bed to find my newly cleaned kitchen in a mess of pots and dirty pans again, and have another stranger to chit chat with politely, at least for a few minutes.
I was not comfortable with an unknown stranger in such close proximity to my sleeping little boys. And I would not myself go to sleep with him still in the house.
Now I was lucky in that my AP was very religious and her boyfriend a youth preacher, so the issues of sex under my roof never came up, and she never went to stay the night at his house, or even visit in the evenings as that wouldnt be "decent".
If her boyfriend is in her home country, she would have to go visit him on her holiday time, you wouldnt let her have time off to visit with anybody really, unless it is Easter, or Christmas etc. As for families and boyfriends coming on holiday to stay at your house, no no. However, if your AP arrives, you like her, she is nice, and it is working out famously, the offer of a loved one, mum/sister/bf for a long weekend would be highly appreciated by the AP. Your home is not an immediate holiday home for APs friends and family. But the stories my AP has told me about other live in nannies/APs that she has gotten to know, and how they have parties and guests when the host family is away, and how they sneak in their finances discussing how much they earn and decisions regards to expenses, etc, I think I have been very lucky, LOL!
Do you actually have to state upfront in the application papers whether bf is allowed to come and holiday (stay as guest at your house) or visit in the evenings if he is in the same country?