I would completely ignore any pretend anger he says he feels for you snooping on his phone
What you found out justifies the means, IMO
he is angry because he has been busted
there is more to this, lots of cheated-on women (and men) often think their spouses just don't have the time/opportunity to conduct a fling
but they do, they squeeze it in somewhere
if there has been anything physical between them, it may have not been very often, or not for some time, but I would bet good money there has been at some point
no "good friend" would carry on dirty texts for 10 years without something to fuel that fire...
he will tell you the minimum he thinks he can get away with
if you want to continue this relationship, he has to come 100% clean about everything, because if you find out things later it will be worse for you
and if you want to call time on him, I would understand that