You are entering into a very dangerous and precarious situation.
You have no way to ensure that she will be honest.
She could twist things around and give you more pain and suffering.
Let’s play devils advocate. She responds to you and says "nothing happened". You don’t know if it’s true or not.
She could be a mean vindictive person and maybe there was an affair and your STBXH dumped her. She can then turn around and play you as a way of getting back at him via you.
And that is just more pain for you.
I have a very similar situation as yours. My H had a 4 year EA that he refused to admit to me. This was before texts and it was all in person. I knew it was going on but he had this attitude that no sex = no affair.
It finally ended and it was rug swept. Never brought up again. Fast forward 15 years later and he’s now having a midlife crisis affair and he’s planning to D me. The OW admitted to me that my H told her he cheated on me in the past with the first OW. She had very specific information so I knew she was being truthful.
That to me was worse than his current affair. He knew all along he was cheating but refused to admit it.
I now believe it was more than EA which again, he continues to deny. Said it was NEVER a physical affair.
In order to R I had to get past it. Other than this one point, he did everything possible to R after his second affair. He knows I believe it was a full blown affair and I am very very positive I am right.
Yes it irks me he isn’t honest. And yes it upsets him I don’t believe him. But I had to let it go. He’s been very trustworthy these past 11 years since his last affair.
For you, I say trust your gut. If you believe there was an affair, there was most likely an affair. And you don’t need anyone to validate that for you.
I hope this helps you. Trust your gut.