You are right to be concerned, and you are lucky you have the opportunity to nip this in the bud, take it from someone in the midst of an emotional affair between my husband and another woman. I wish I had caught it sooner it is destroying my relationship with my husband, and the longer it goes on the more it takes root. In my case it started out as a 'friendship' between my husband and an 'understanding' but manipulative woman. Make sure that you are around a lot more, be visible do not give them the opportunity to spend time alone, treat him well and make a fuss of him if he feels unappreciated and misunderstood this can be the trigger for an affair. I know this sounds unfair, like a reward to bad behaviour, but it will lower the risk. Try to involve yourself more in his friend group it will make it more socially difficult for her to start something with him. You are right to move for him to cut contact, but by the sounds of things it needs to be communicated more clearly to her. If she can't add fuel to the fire then it will fizzle out, now is the time to put the effort in and be vigilant. You need to work to provide what she has been providing so that you are the person to meet these needs in future he is upset about losing the friendship because she is meeting some of his needs, find out what those are and you will be able to phase this out.