JR, I agree that's not an ideal thing to find out. Try to approach it unemotionally (easier said than done) and see what you find out. Better to try to sort through it first before jumping to any sort of conclusion.
Curious how she brought it up to you? If there was something nefarious going on, it seems odd she'd offer the information directly to you. Who knows though. My W has "hidden" things in plain sight, so it's difficult to know what they're thinking when they do stuff.
Regardless, keep your cool, get some intel, and go from there. Here for you brother, sorry about the latest twist.
Me39 M11 : T13 D9 BD 5/31/16 In House S until 6/21/17 Divorced 10/5/18