In this case, the woman should be interviewed by the Bishop or Stake President. After prayerful consideration, he may wish to question your husband on the matter. If the husband denies everything and states that he was stalked etc. and they have prayed and believe him, then that is the end of it.
If they do not believe him, then they may hold a court.
If the affair really did happen, you have not lost your eternal marriage. There is still forgiveness. I know of members who have had similar things happen and have fully repented and now hold callings.
I also know of a women who was "someone" went to the Bishop as accused her of infidelity. The Bishop interviewed her and found it to be groundless. She was innocent.
If perchance the man in question has done this...but gets away with it, well the Lord does know...and like it was written he will "get his on the other side." As far as your sealings go, they are still valid and the wife/you will still be given all blessings due her/you.
From what I have been taught, when one person makes it to the Celestial Kingdom and the other partner does not, the worthy person will be given to another. I have heard some complain about this because they "may not like the person to whom they are given." I would think they would still have a choice to say no, but what better match maker is there than a loving Heavenly Father, who knows us better than anyone?
As far as the marriage goes, there always has to be trust...and of course you deserve to know the truth. But these things are personal between husband and wife. In some case I have known, the husband was guilty, they worked it out and are still together. In other cases, they were not able to resolve the issue and ended the marriage. AGain, it is a personal thing that should involve much prayer and fasting, as well as counseling with the Bishop. If it were me, I would ask for a blessing, because of the stress.