OK. Here’s my take on this. It’s OK for them to see each other. Your friend should know up front that this guy is on the rebound and will most likely not be a permanent relationship between her and this guy, although I’ve been with my guy for a while and we are looking forward to a future together.
However, your friend should know that should she sleep with this guy or have any appearance of having spent the night with him, the STBX could come back and sue her for Criminal Conversation (CC) and possibly Alienation of Affection (AA), should she have known him prior to their separation. With CC, it doesn’t matter whether or not they are separated, who asked for the divorce, or if there is a separation agreement in place. The only thing necessary is that they are still legally married and your friend and this guy have had sex.
I’m in trouble with my BF’s STBX because she is unhappy that he found someone so quickly after they separated. She has jumped to the conclusion that we had an affair all along, even though it is untrue. She asked for a divorce after they had been through several marriage therapists and nothing had worked. He agreed and moved out, secretly relieved that she had been the one to ask for the divorce. When the STBX found out about me, she has tried to get us both fired, has threatened me physically, and has threatened to sue me for AA/CC every month since he and I have been together.
As far as legalities go, there’s no such thing as a legal separation in this state, although drawing up a separation agreement is a good idea for the divorcing couple. It’ll help negotiate any financial arrangements prior to the actual divorce and the divorce could potentially cost them less in the long run. A clause within the separation agreement which says that either partner will not pursue litigation against a 3rd party romantic interest could somewhat protect your friend up front, should she decide to become intimate with this man. The clincher with this kind of clause, though, is that asking for it in a legal document can give the appearance that AA and/or CC has already occurred and the request for it might set off the STBX.