Can I ask his ex if he cheated on her?

I recently found out he cheated on me (just happened once). He said that his ex broke up because she was very jealous, but now since finding out he cheated on me, I wonder if he did the same to her and I want to know what the probability is of him cheating again so that I can know whether to stay and put in the effort to let go of the past or move forward to avoid future heartache and regret that I didn't leave the first time.
Can I ask his ex if he cheated on her?
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