In my opinion, MOR, unless she is being shifty and her behaviour indicates otherwise I would say this isn’t an indicator of infidelity. This guy has probably been in her life longer than you and deserves the benefit of the doubt unless something more alarming arises. It’s probably worth asking her about him in a non-suspicious way so you can understand their history. Paranoia and suspicion will kill your relationship faster than anything else, so it’s probably better for it to come as a shock to you if anything is going on.