Most Helpful Guy
You don't trust him? Would you be upset with him for receiving it or upset with her for sending it? It can be seen as a compliment that you have a quality man that others desire. Just talk to him about it to see if there's anything behind the message.
Most Helpful Girl
I would Definitely Be Annoyed that this 'Girl he works with' has his cell number. And Okay, with probably good reason, they work with one another, I accept this.
However, for her to just send This sort of 'Message' out of the blue, I would face him down with it, ask him what is going on here, dear, and tell him if there is no hanky panky going on, she need not Say 'Morning' with an emoticon and Kiss next to it, Inform her to Keep it lite and semi sweet In the Office and Not... After hours and before the doors open the next Morning.
Good luck. xx