I can't help viewing this scenario through the eyes of the guy's mother. She most likely took one look at the girlfriend and fell into deep despair. If I were her, a number of things would've gone through my mind, namely that I had to have done something, not done something or missed something if my son thinks that a 25 year old cashier is a viable mate. No one that age should be just a cashier; doing it while hustling towards a greater goal is the standard. The fact that she's just a cashier would speak volumes to me and is a huge red flag. I would've also spent countless hours agonizing over the future grandchildren, e.g., how could she possibly raise them properly given her upbringing and world view, as well as those around her; the negative impact her family would have upon them; what dysfunctional and/or raggedy habits are they going to pick up and become socialized to consider as normal; do I even want to see this tragedy unfold before my eyes; etc. To be sure, that guy's parents have been scrambling and drinking heavily since they met her.
OP, I know your heart is in the right place and I commend you for your efforts, but hooking her up with that guy is, IMO, tantamount to bringing him and his progeny down in order to lift her up. Is that fair, particularly given all that he and his parents have invested in him? They're doing all of the right things, so why should they suffer? Why should his kids suffer? She's clearly unfit. Her village is unfit. Nothing good would've come from a union between them. It would be better for her to find a nice, honorable and resourceful garbageman, postal carrier, plumber, social service worker, etc. that will allow her to live decently and, if she matures and figures some things out, afford her with a position to reach back to help her family and friends out of their generational ruts. IOW, she would do best with someone who is no more than two steps above her current socioeconomic status; anything more would be a grossly, possibly disastrously, uneven yoking.