So, how did you come to see these messages? Sounds like you were not supposed to see them. It could be your mother is interested in the guy and saying things she might not necessarily mean to keep him interested in her.
If you'd like a male view, it could very well be like Jens said and your mother is trying to keep the guy interested, or your mother has fantasies in that direction. You'd be suprised what sexual fantasies some people have they wouldn't tell anyone but an anonymous person at the other end of a message.
I think the important question is how does the thought of her seriously considering it make you feel. Interested, disgusted, curious? I'm curious about your answer.