I think you should casually mention what you saw on his phone in a non-accusatory fashion and simply ask who she is.
He has cheated before, so I think it's fair on your part to be a little suspicious.
If I was personally in your situation I couldn't just sit on it and wait for more 'evidence' to surface. This could be a flirtation that hasn't gone much further and I think it's best to try and establish what is going on now rather than later.
It's up to you how you handle it OP, but I think it's suspicious enough to warrant follow up on your behalf.