Hi, I underatand your concerns.
Please allow me a few moments to prepare an answer for you.
I think it would be nice to talk with him about it your concerns as a friend. Try not to be eemanding or negative since he could just confuse that with you being jealous. Rather explain to him why you are worried. Let him know that you do hope things work out for both of them, but you can see things a little differently since you are on the outside looking in and hope he can take a step back to analyze things in order to make the right decisions. Talking from your heart is the best way to go here
since that is how he will feel your sincerity. Please let me know when you are back online ifI can be of further help to you.