it's a good idea to try to help them out a little. I do understand the pride thing, so I think outright gifts of money are out. I also think that her showing up back at home with boxes of stuff isn't going to work either. Not that you can't buy her something, but I'm thinking spending that money on experiences is going to initially be better.
Having her bring a friend is a good idea, but you'd have to talk to the parents first. I'd keep that for a future goal and just try one-on-one for now as it's just easier.
As far as what to do, I'd ask her for suggestions. Doing what she wants to do will always work out, but if her ideas are too modest (like searching for recyclable bottles), then give her some ideas.
As far as her grades, I would use those as the excuse for taking her out. Tell her parents you're proud of her and you'd like to reward her for her hard work. Can't argue with that.
Giving dad a job and paying him a nice wage would be a good way of thanking him.
I had a similar experience a few years ago. My daughter became friends with a girl that was clearly poor. I brought her back from a party once and I'm pretty sure they were squatting on someone else's land in a trailer with no utilities, up a rocky ass road in the middle of frickin nowhere.
I finally was able to connect with dad and things seemed like they were going well when they ended up moving, so never got a chance to do much to help her out. She was always happy to see me though because I paid attention to her. All girls deserve a pedo in their life.