Your partner hanging out a lot with your best friend out of no where without you, being vague about details (on purpose, since he was trying to make him jealous and angry btw), and then you find them together like that... even in a secure relationship, you're allowed to have doubts when someone is acting strangely
Yeah but there are other reasons maybe your partner wants to do something nice for you in which your best friend would have details about it and if it's a surprise you would be vague about it. Again that's not what's happening here but there are multiple other reasons for a situation like this cheating is only the first thought of someone that doesn't have trust in the relationship.

I'm still on his side. That situation was SUPER compromising and they've been acting extremely suspicious + the set up was perfect for this level of a misconception. Being betrayed by someone you care about that much and have grown up with hurts 1000x more so I don't think his reaction was unjustified.