if she is your best friend why go to your husband? and not you?
BE CAREFUL this is a sticky situation,
if she is having problems in her marrage and yours is all happy and fine she may be getting closer to your husband kinda wishing he was hers! if you see what i mean,
as she is your best friend she obviously knows you very well and may look up to you and maybe look up to the relationship you have with your husband and feels jealous possibly that hers is a mess,
getting close to your husband isnt doing her, you or anyone else any good!!! id put a stop to it, because if nothing has happened it may do now,
and the fact that it has been hidden from you IS a problem,
im not saying they are having an affair, but she will probably feel drawn to him coz he is everything her own husband isnt! and the more he is there the closer they will get,
be carefull talk it out and to be fair if you feel your marrage is at threat put a stop to it,
cant she talk to you about her problems?????