Your husband can cite another person in divorce proceedings, however, he must have proof or reasonable suspisions that you have committed adultery.
You can defend the allegation, and the courts would look at all the evidence presented.
Get yourself a good divorce solicitor, and they will advise you properly.
Suing for slander is an extremely expensive business, your friend must feel damned by these allegations, but taking your husband to court may only inflame the situation and make for juicy gossip.
Again, seek legal advice asap.
Good luck!!!!!!!!
J x