NEED SOME SERIOUS ADVICE

My boyfriend of 4 years (on and off this whole past year) has this friend who they have been close for a long time. His friend literally was messaging me and flirting with me and told me not to mention it to my boyfriend. Of course i told my boyfriend anyways and they stopped talking for a month and are now friends and hang out like it never happened??? It honestly pisses me off and idk if it’s about respect or what. Help me understand this logic?!