Your wife and potential girlfriend are flying towards a cliff at a million miles per hour. When they hit bottom - and they will sooner or later - the fallout will impact you, your wife, the girlfriend, the GF's husband and any children involved as well as lots of other folks (family, friends, work associates, etc.).
Beg them to wait. They are cheating. Yes, when one is so in love and so hot for each other it is so painful to wait. Do it anyway. They can start their relationship with integrity and trust with each other and with you and with the husband if he is still in the picture. How long once the love and sex stupidity wears off (and it will) will it occur to your wife or her girlfriend, 'Can I really trust this person? After all they had an affair with me?' How long will this go before you wonder 'Can I trust my wife? After all she is having an affair with this person?'
The girlfriend needs to sort out things with her husband first. Yes, that may take time. And maybe he is a dick and she should just dump his ass and leap headfirst into your wife's arms. Or she may want to see if they can salvage the relationship - and still jump into your wife's arms with the husband's knowledge.
I personally do not think it is your place to tell the husband. However, if asked, I would not lie. Tell your wife and her girlfriend that. You will also not assist them. You support your wife's love for this woman but not her cheating. And tell your wife how disappointed you are in her. Where is her good sense, good judgment? That may get your point across like nothing else.