Should I mind my own business?

My cousin is also one of my best friends, and I would do anything for her. She has been with her boyfriend for almost a year and I have not spoken to her about something important because I'm not sure if its my place or not. Please advise. They seem happy together, but I can't help but notice that she seems to pay more than her fair share. If they go out for a whole nite, she will pay for taxis, niteclub fees, etc. while he hangs back. I have seen him pay for some drinks during a night out, but she will go to the bar equally as much to get some more rounds. When they go shopping together, he never treats her to anything, not as much as a bar of chocolate. Yet sometimes she will pick up a new t-shirt, hair gel, other stuff that guys need - and he has no problem accepting these. He earns as much, if not more money than she does, yet he never seems to treat her the way a girlfriend should be treated. He treats her respectfully in every other way, yet I feel when it comes to money, he should stand up and be a man. Their relationship seems to be getting serious, so I wouldn't like to think that if they ever got married, she would have to try to survive on her own wage while he holds back and keeps his money for himself. I love her very much, and I can sense that the way he acts does not suit her, yet I don't think she will speak up as she doesn't want to cause conflict between them. What would you do? Or should I even try to talk to her about this? Its a hard subject to approach.