i'd find out politely what happened that time. don't go grudgingly, or rudely. just casually speak 2 her & don't start out w/ that. just start w/ casual conversation & blend right in2 that. don't play the blame game if u really want 2 find things out.
u never know if they may not b just friends & nothing more. IF however, he deceived her, then blame him 4 lying; (many men DO deceive). just b on friendly terms w/ the woman & don't get suspicious yet; it may very well b innocent 4 all we know.
i used 2 have upstairs neighbors yrs ago, & i was a family friend. i was able 2 b trusted alone w/ the man & their kids, b/c i had no interest in anything more than friends & that's it. it's nice 2 B able 2 have built trust in relationships by proving trustworthiness. staying pure IS very advantagious