If this couple is decided upon separation and divorce, then your friend needs to let go and gain some acceptance and stay out of his stbx's personal life. It is up to her, not him, whether she dates. And her cell phone is not for snooping.
It is really irrelevant what her dream life would be. Her wanting it doesn't make it so. D is a business arrangement, and he should work with his L to secure an acceptable business arrangement.
In the meantime, he should quit trying to control his stbx, decide his own boundaries, and enforce them. So, what to do in this particular case?
STBXW: Hey, is it OK if I date? Friend: That is really your choice. I prefer that you be honest with me about it.
He could, I guess, let her know his boundary -- what he will do if he dates. But, determining and enforcing that boundary is really what his focus should be, rather than her and her choices. Certainly he should structure his life so that it is irrelevant whether or not she is dating. (This is consistent with an agreement not to have one's date in the shared household.)
That being said, he is pretty clearly not anywhere near being emotionally D, nor is she. They are evidently still living together. If they are serious about D, I'm not seeing it.
But, assuming they are, the best advice I could give to your friend is: Get an L. Determine who will move out. Make the move happen as soon as possible without hurting your position in the final settlement. Separate your personal and financial lives as quickly and cleanly as possible. D is business, treat it as such. And let STBX's personal life be her personal life.
