As far as your description, I think you're friend's main issue is with class differences: (an example)
I would suggest he really get to know her and her family before proposing. If he proposes and the father/mother do not like him then there could be tension. also, if she has way more money than him, he might feel like a burden or that he needs to make more money or that he isn't good enough, etc. and she might feel like she has to support him or that he relies on her solely. This could lead to problems as well.
If the two people get to know each other and actually do love each other, then love will conquer all that silly stuff I just posted. I am not a 'love expert', this is just my opinion....