Why are you worried about "his" motivations? She is with you, not him. What she did with him is in the past, in her past.

Are you unable to respect her because she did X with him and how she allowed their relationship to be on his terms ? This doesn't make her a bad person.

Apparently she loves you and decided to open up to you about her past. How can something like this make you question your ability of respecting her?