Depending on when he told her he didn't love her any longer, it's possible she's still in shock. If she's in contact with the OW's husband, it's also likely that she's had lots of fresh shocks as new information came out in drip feeds.
It sounds as though the catalyst for all this was the OW's husband finding out, not the relationship petering out of its own accord, or because your friend's H ended things. Some of the lies about their physical relationship are being maintained by your friend's H to protect the OW, who is no doubt lying to her husband because she thinks he can forgive emotions, but not sex.
What people always under-estimate though is that discoveries of any kind also involve shock for the people who were found out. Consequently, what they feel, say and do is often as unreliable as the behaviour of the faithful partners who are also in shock.
So her H might be re-writing history about a longstanding loss of feelings for your friend that is simply not true, because his emotions are being clouded by the new feelings (note new, not deeper) for this woman and the loss of her. So she's right to be sceptical about her husband's lost-love claims towards her and she's right that this might well pass, but wrong to wait around until it does.
If she wants to save her marriage and believes she can forgive, curiously the best thing she can do now is to end her marriage and ask him to leave. He will never respect her again if she stays with him and more importantly, she will never respect herself if she makes these trade-offs.
Yes it's a risk that he will go off to be with the OW, or nurse his wounds when the husband takes her back and she chooses him and not her lover, but statistically there is a greater probability of your friend's husband coming out of the fantasy affair stage when he is living by himself and deciding he wants your friend back. For her to take him back at that stage will require total honesty and a great deal of work on his part, but with any luck she will be out of her shock then and will have created a better life for herself without him - and will say no.