I think if she is happy with it, just leave her to it. If she begins feeling uncomfortable with M's GF being there, I'm sure she could bring it up to M in conversation and they can work it out. If she comes to you directly for some help and advice, then is your time to get your strong word in if you must. =P But everyone seems to be happy with the situation as it is apart from you. Do you talk to M's GF at all? Maybe lightly suggest that "wouldn't it be great for M and my GF to get some alone time, how about me and you go out for a meal that night and let them be together?"... Just my suggestion. =]
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