Of course I wouldn't want their friendship to end. I just encourage a level of respect towards the girlfriend's well being regardless if she knows her or not.
I had an affair with a married woman. My biggest regret is that I harmed what her husband had with her without him ever knowing it. I didn't harm what she had with him..she did that.
I am not equating this situation to cheating by any means but consider patience to be displaying respect to what him and his girlfriend are growing. Give that plant some time to grow and see what comes of it. Be the best friend you can be in this and who knows