If the husband is fairly certain that penetration has occurred during foreplay but the wife is sure that it has not, how can they resolve their understanding?
Every one from them should act according to his knowledge. If husband is fairly sure that penetration has occurred, he then needs to perform Ghusl of Janabah because by penetration, he has become a Mujnib. If the wife is sure that penetration did not occur, then she is not in Janabah because of that foreplay unless she reaches to orgasm.
Wassalam.