Boy this is interesting hearing this from the guy's perspective.
I admit I've slept with some men that I knew there was no future with: too young, Love Avoidant, others. But I don't think I've ever slept with someone if I thought I would never be interested in a relationship and suspected they would ( and would get hurt).
Are you really sure you wouldn't ever be interested in a long-term relationship with her? If you wouldn't, do you honestly believe she wouldn't get hurt? Or are you fooling yourself by thinking you won't get involved if you would?
Now she's a big girl, but just don't misrepresent yourself. If you're only looking for fun with no commitment be clear on that. I'm forever grateful to my Love Avoidant friend that I was seeing again recently, for having been painfully clear when I first met him that he "doesn't DO relationships ". The upfront warning allowed me to protect my heart and avoid unreasonable expectations with him. (Which was important because otherwise, he might have broken my heart - of all the men Ive dated since my divorce he's the best fit physically and intellectually - but broken sadly. )
It's good to hear that the married guy lied to her - very common story, not her fault she got taken in. And sad that her exH was abusive.