Rather than a party, per se, perhaps your friend’s wife intends to hold a secular version of the Irish Catholic tradition of a wake, which is a vigil over the body of the deceased that is held before burial and at times includes some festiveness in honor of the deceased. If your husband has been invited to offer remembrances of his friend, it does sound as though the event is not intended to trivialize the deceased’s life.
I do not know of any Church document that would prohibit you and your husband from attending, and your husband from offering a eulogy. While Catholics might wish that a lapsed Catholic had been given a funeral Mass, attending this event might give your husband a sense of having honored his friend’s final wishes. If the “party” becomes too raucous, you and your husband can always leave quietly after offering your sympathy to the bereaved. You can also arrange with your parish to have Masses said for the repose of the soul of the deceased.