It is possible that you may not be able to provide any support to either of them. Not so much because you are part of the "problem" but simply because you are too close to the situation. You can not be the supportive friend to both of them at the same time. You may wish her well, but ultimately your priorities lies with your relationship with him, correct? Even if are able to fully support her and her feelings, you should consider her position. It might be possible that she doesn't want your support because she doesn't know how to handle it.
I think this is where I would choose my path. Would you be willing to wager your friendship over the possibilities of this romantic relationship? Where does it sit in terms of your relationship with your husband (also)? If you value the friendship more, perhaps you guys can put things on hold till they are through counseling. That way they can both work through it without any extra influences.