Household are going to have a removable top for the water closet portion and likely to fracture with road stress. An "RV" or a marine head are the options. For your friend he should take his wife to a RV dealer and see how they work.
It is quite likely that your friend's wife has no desire to be an RVer and if the toilet problem was fixed she would find another problem to take its place. Often when a person provides a response that seems illogical it is because we take their response at face value when it is better to ask more questions and learn what the real problem is in their minds.
That is why a salesperson learns to ask the question "if we fix this will you buy it or is there anything else that is a problem or concern" or words to that effect. If the person says yes then the salesperson knows they have trapped the buyer and have a done deal.
