my friend's ex talking stage just DMed me

Okay so my friend (lets call her purple because that's her fav color) had a talking stage in the summer over Whatsapp, he liked her more than she liked him and he is quite immature, but they fell out badly because he was "drawing" (as he put it but it was more like tracing) lots of girls and posting it on TikTok (for example: Sabrina Carpenter and others, some he knew in person) calling them beautiful and stuff and then he had the audacity to ask purple to draw her and post it , and she politely declined yet he posted it anyway which obviously annoyed purple so she ghosted him for a while and he started asking what he did and she explained to him and instead of apologizing he said "let me think about this" (that's what she told me) so she blocked him , however just now he DMed me on instagram (i didnt even know we followed each other and I didnt originally recognise his account because why would i) but he said "Your (purple's) friend, right?" and i was like "yea why" and he started talking about how he was blocked on whatsapp and in my head i was like yep i know who you are now, ur that one who doesnt listen to her 🙄 but i wanted to see what this was about although it was a fairly predictable conversation : "can i ask u for a favor" "what is it?". (in short) "Tell purple i am sorry, she's amazing, I still love her+I said things I regret" . i felt the need to tell her about this and be honest with her so i told him that i could tell her, but right when he thanked me i started getting second thoughts because purple is going out with this guy who is much better for her and I don't want this to disrupt the potential relationship that could bloom between them, but i still feel like i owe it to purple to tell her and be honest but i don't want to complicate things for her, and ive never been involved in guys and friendship drama and ofc I will always choose my bestfriend and would never sacrafise our friendship for some guy because i can just tell she is the best friend i've ever had and we even established between us we are soulmates (platonic) because we just get each other but Idk what is best to do in this situation?? I feel like im overthinking it and just need some advice