best friends boyfriend??

so my best friends boyfriend got an app where you can text or call people and it changes your number, well he texted me on that and was calling me cute and talking about how he will miss me when me and my boyfriend move 15 hours away. i knew it was him because my boyfriend and him surprised me and my best friend by making dinner for us and he said something about it while texting me. i didn’t flirt but i did text the number enough to get proof. i told my boyfriend and he’s not very happy about it. how do i tell my best friend?! she’s like head over heels for this dude, and we’ve all been friends for a while so it’s really weird for him to do this. the texts weren’t super flirty or asking me to hook up, but if my boyfriend did this i would want to know. i don’t want her to be mad at me and i really don’t want to lose our friends:(