Would you say "te quiero mucho" to a coworker?

I was looking through my boyfriends phone and found an outgoing message that says te quiero mucho to one of his lady coworkers. He denies that they are friends, so it really bothers me. Most of their exchanging text messages were in spanish (I dont speak or understand spanish), so I couldn't make most of it out. Any thoughts? Ever seeing that I'm kind of worried that he's cheating on me and seeing this girl.