Caught him textinga girl need advice

I just saw texts he deletes them for her because it was a brand new text this morning saying hey babe. I already caught him talking to her before. Anyway he woke up so I couldn't text her. I was going to text her on,his phone saying this is her gf he lives with etc. Or I could pretend to be him and say I dont think we should take anymore to her. Or should I call her on my phone and talk to her. He says hes just helping a friend. I told him its inappropriate and disrespectful. What should I do? Advice please