If this girl is happy with her boyfriend , then back off and leave things alone.
If she isn't happy with her boyfriend then that is up to her to do something about it .
the worlds a bad enough place as it is, if theres just one nice thing about it leave it the hec alone and don't mess !, if she ever leaves him (or he dumps her) you have a chance. of course you give no circumstantial information (like age would help)