Okay first, Intent matters! If you're completely platonic then I don't see why you can't be that way with your bestie. Its not cheating!!

If your husband is uncomfortable thought, he can openly comminicate and maybe you can explain things and you guys can come to a mutual agreement. But saying its cheating when there's no romantic connotation to it is ridiculous. We all have friends and we all need friends we can be open with other than our partners tbh.

Also, did he snoop or is it okay for you guys to go through each other's phones?

I think there needs to be very open and honest conversation in an Adult way. He can be jealous and the sharing of intimate details...that can be a uncomfortable thing for many. So talk with your partner OP. But its not cheating unless there's romantic feelings involved. We all need good platonic friends, only thing is its hard to maintain them as adults. You're lucky to have a good friend.