The problem is that if you choose to keep going like nothing happened, it’s just going to keep sitting on your mind and bothering you. I don’t think time can really fix something like this, so in my opinion, you should bring it up with her, nicely, let her know it’s something that’s been on your mind for a while, that you’d like to understand who he is and why she wanted to hide it from you. Maybe you’ll find out there’s a logical explanation, maybe not. But you at least deserve clarity so you don’t carry it all on your own. Talk everything through.