That's CHEATING! If you just said that they were talking about whatever via text, I'd say that's not cheating, that's having a female friend. Talking like THAT on the other hand with a person who is not your significant other is cheating!
How does your friend know about this? Did she look at his phone? I hate to be devil's advocate but 1) if she has to check his phone for this kind of thing, that's a problem and 2) how do we know that he didn't just say that to cover his tracks just in case she's seen the previous text messages?