help pls

okay my MIL got a new phone but it ended up not being compatible with her service so she brought it to me, asked me to reset it and then go exchange it for her. i said sure! so i just got around to turning it on and resetting it for her. You know on an iphone if you slide over to the “search” page sometimes it shows random pics from your camera roll, sometimes it shows music or recent contacts etc etc. well i slid over to search contacts and on there was screenshots of a text. I WAS NOT SNOOPING. God literally just put this info in front of me.. obviously i was meant to see it. anyways it was texts between my MIL and my husbands ex gf. i guess when we first got together she was still coming over when i left for MONTHSSSSS. she said “is ____ at work? i’m gonna go see him and talk him to getting back with me” then asking if i had left or i was home so she could come by and see him but i guess they didn’t text so i didn’t find out and that he called her and told her first when we found out i was pregnant so she didn’t find out somewhere else and so she texted my MIL and told her. before we even had a chance. i’m so hurt and disgusted and i feel sick and like my phone entire life is a LIE. we have a whole child together and we’re MARRIED now. like wtf.... do i say something to him?! or do i just carry on with my sad unfortunate fucking existence? or do i just leave without a reason? i don’t want them to make me into the bad guy and think i went through her stuff buttttt i literally didn’t like i said it was there when i opened her phone the lord works in mysterious ways... but yeah what do i do in this situation?