First, there will be a doctor on board. The cabin crew/concierge can put them in touch. A feeding tube (maybe semi-permanent) is an option. Then liquid food can just go down the hatch and bypass the gag reflex and teeth problems.

Re the dad situation - he may be genuinely unaware of HOW to do the feeding stuff - it’s previously been a nurse’s job in his professional life with all the status ambiguities involved.

Imo the ship’s doctor and nursing staff are the best option rn. They’ll know what to do and probably have re-hydration supplies on hand anyway. Please tell us of any help we can give. We’re all here for you.