Since there's no evidence, and her story makes sense, then there's not much you can do but accept what she told you as truth.

However, let her know that you feel she is hiding something and she damaged your trust in her.  Ask her if her needing closure with her Ex was worth damaging your marriage.  

Also, let her know that she better save their messages if they contact each other again.  No more sketchy conversations with no evidence trail.  Better yet, I would ask that she block his number.  This shouldn't be a problem since she has her closure now, right??