Do you still have access to her phone? Is she willing to let you check her phone after your discovery. I realize she can find other discreet ways to communicate if she really wants to. After confronting her with texts did you ask about why she said she was "afraid" of him? This is the easiest way to bring up the discussion by directing the interest due to your concern for her safety and not the sex directly.
If you're going to get her to talk you need to be cautious. She cried, she denied anything happened, she knows how those texts came across to you, she was caught taking risks and probably feels some shame. You're not going to simply and suddenly flip all those feelings and emotions into a positive sexual one. She's going to be highly sensitized to how you respond to her. I would guess that you coming right out and telling her that you've always wanted this won't be enough for her to be fully convinced.
Maybe you could try taking her out to dinner and bring up the feeling "afraid". Work from there on cautiously revealing your cuckold desires. At some point, talking about it has to become less filled with anxiety and more open and frequent. If you really want what you want it's worth being patient and cautious. Right now you should get past this first hurdle about knowing she knows that you know.
Please don't take any of this as advice. It is just my thoughts. You know the situation best. From what you've written yes, I think she has had sex with him.
