Have you considered that maybe this "friend" of yours has changed anything about your behaviour to lead your BF to think that you are misleading him? With that said, i cannot imagine a healthy relationship where either of the partners has complete access to the other ones' privacy, though i know people that make it work. Up to you.
Also, how do you think your bf feels about the fact that you are looking for other men/boys to flirt with you, regardless of flirting back. Just be honest with whatever goes on, while still keeping your privacy. I don't know your boyfriend (maybe you feel forced to lie because of him overreacting) but you should've told him. Would you want him to tell you if he's chatting with a girl on snapchat but to rest assured, it's all platonic on his end or would you have rather found out some other way? Staying silent can be a form of lie.
PS: i'm in the dude's corner because i see you already have a lot of support. However, he should not want to go through your phone.