What is the ruling regarding a committed husband's abandonment of prayer while traveling, and his bringing prostitutes to the hotel and fondling them to the point of ejaculation? Is this considered zina (fornication/adultery)? And what is the remedy for such behavior?

Neglecting prayer to the point of abandoning it is one of the gravest sins, and it can even reach the level of disbelief (kufr). As for your husband's actions of fondling prostitutes during travel and committing adultery, this is forbidden even if it doesn't reach the point of penetration. He must repent to Allah Almighty and abandon this behavior. You should advise him. If he rectifies himself, then good; otherwise, leave him so that you are not dragged into his actions.