I've posted before about this and would welcome advice on a new development. My 53 year-old DP lives with his 18 year-old son and the boy has requested that his 17 year-old GF moves in with him and DP has agreed. They're both students with part-time jobs. DP works away all week and his son treats the place like a hotel - never doing any clearing up despite repeated requests from his dad. DP gets home on a Friday night to a scene of utter devastation and I shudder to think what state the house will be in if the GF moves in too - she's VERY dirty and the house will become a complete health hazard.
Currently DP and I only visit eachother because we live 4 hours apart - so I see him for 1-2 nights every other weekend. Recently I'm the one who's been doing the lion's share of the travelling though because DP is stony broke and working on Saturdays. I just don't fancy the thought of spending my precious time with him in his home if it's going to be used like a hostel and think I might have to accept that DP just comes to mine or we meet halfway and stay at a B&B. I have a DD of 9 who's usually at her dad's when he comes to visit and we can really spend quality time together.
My feeling is that the son and his GF should go and rent a room locally if they want to be "independent". It would only cost them around £35 per week each and might be a good thing for them too, teaching them the value of money etc. DP and I are planning a life together, once the son is more independent, and the hope is that he'll come and live with me. I know DP, as a divorced dad, has all sorts of guilt issues about whether he's done the best for his kids (he also has a DD of 21 who's away at uni and very happy)but I think "enabling" his son in this way might not be the best thing for him either.