Would you be PO'D if some random girl called your bf daddy?

Okay so alittle backstory, this girl was I think one of his highschool classmates/friend and a best friend of one of his friends from his group in hs, now they used to go to the same college before he left it. He says shes "weird" and that they never really hung out. I know they don't for sure now. But anyway they were friends on Instagram and their conversation pretty much consisted of memes and stupid jokes that relate to stupid memes. She would send him little hearts and ":3" faces and shit, whatever I guess... But then he sent her a meme that was so dumb and i didn't even get it honestly but it was about Mario Cart and said "wow lite" (wtf) and she sent him a heart, liked the photo and said "thank you Daddy" and then he liked her response.... I was so mad I was on his phone picking music and that showed up and I threw his phone down, he tried to explain to me that she was stupid and that they had never hung out just them and that he didn't even know her like that. I started to get emotional and then so did he. I don't honestly think he cheated on me with her but idk, would you be mad?