found a dress and dirty girl underwear at home

So my friend (I call her friend, met her once and we've kept constant contact since) well her and I got married right around the corner from each other(one month apart) Whenever she has a problem with her guy she will always contact me. Since she just moved in with her spouse after married and I lived with mine for a while before marriage. She feels like I can always give her good advice, well she called me crying because she was cleaning her home and she found a girls dress and some used underwear 😫 (gross) she ofcourse flipped and honestly I am a straight up person and will tell her what's on my mind. She immediately started making excuses for him saying they could of  been from before. But who the fuck keeps someone's dirty underwear !? Specially once packing to move in with your now wife!? Well, she talked to him and her words "he didn't seem like he was lying or nervous he right away said they're his exes and he just forgot to throw them out" I call bullshit !! He was talking to his ex like they were bffs not long ago until she had him block her. Idk am I too untrusty ? If it was me I'd raise hell because I'd 100% believe cheating. What would you ladies do ? How would you handle the situation? They both work different shifts so they're never really home together but to sleep for a few hours. I suggested her getting a camera but her response" I know he isn't lying, I wouldn't have married this man if I didn't trust him"