Sorry - epic tale here.
My DD is 31 and a single mother to three ND children.
Her partner at the time she was expecting her first child, kicked her down the stairs and out of the house that she was paying for when she was 20 weeks pregnant. My daughter had a large overdraft, whilst he had accumulated a very considerable amount of money as he had persuaded her to use her salary for the bills whilst he kept saving for 'luxuries'.
Her next partner contributed a paltry monthly sum to the household all the time he lived there - I didn't know the amount until he had gone. He left her with an additional two children. We found out that he was a fantasist, convicted arsonist and regularly used sex workers.
DD has been on her own in her 4 bedroom house (with mortgage) for just over three years. None of her children have asked about their fathers - neither of the fathers pay anything towards their upkeep or see them.
A couple of weeks ago my DD told me that she was seeing someone (this is during the day when her children are in school). They meet at an out of town outlet centre. I realised she only probably told me because she wants to take the relationship further and see him on evenings. She probably wants me to babysit. I've not been asked yet because my other, younger DD's daughter has been in hospital for a couple of weeks and I've been visiting every day that I can. This week my older DD's youngest is in hospital for the week. So when things settle down I'm expecting DD to ask if I can have the children at night/overnight so she can see this new 'chap'.
I know his name and where he is from. I asked a friend who lives near by if she knows of him and she did. She said to tell DD to steer clear. Apparently he plays computer games all day and night, doesn't have a job, has lots of children he doesn't support, and lives in an HMO. He has told my daughter that he is a wrestling promoter 
My DD is autistic and gullible. She believes everything anyone tells her. She appears to attract 'bad uns' for want of a better expression. She throws herself at anyone who pays her the slightest bit of attention. The bigger problem is that contraception does not work for her - she has been pregnant on contraception three times (injection, implant).
I really do know how this will end (pregnant and on her own) and I want to tell her that I will in no way help her to take this relationship to the next level. But if I do that, I know she will look at me in disbelief, she will burst into tears, and say something like 'don't you want me to have a life and be happy', which of course I do, but not with another 'waste of space'.
I will in no circumstance facilitate this situation.
How can I tell her that without falling out with her? I know she will cut me out if I refuse to help. I look after both my DD's children so they can work. I know my DD will cut her nose off to spite her face and end up in a mess. How can I prevent potential damage to our close relationship? I want to prevent her from being hurt again by another low life man.