Not necessarily unrealistic, since we do not know if the student graduated college debt free, with a high paying salary and low living expenses. Lots of college graduates earn high salaries coming out of college. My first two kids graduated undergrad with zero debt. The first child got a $10K signing bonus coming out of college and lived at home rent free for 4-years, making over $50K per year (this was 12-years ago). She was getting refunds throughout college due to Prepaid college fund, Bright futures and other scholarships.
There are “many” parents who finance their kids education, partly with funds from parent-plus loans. They do so with the explicit expectation that their kids will payback part (or in whole) the parent-plus loans over time, t. A friend of my second child, had over $230K (!!!) student loan debt upon graduation, most of which was parent-plus loans. He pretty much financed the entire $60K+ COA per year through loans. He told my son that his parents expect him to payoff the entire loan balance over time, since they had other loans related to their other two children.
The OP had an agreement with his/her son for him to repay the 5th year. It appears she was willing to forego/delay those repayments, until he/she found out he was more than capable in meeting those obligations, despite son’s protestation to the contrary. IMHO, the son needs to repay the parent for the cost of the 5th year, per the verbal agreement. I also fail to see how this is a hoax when many similar situations to what the OP outlined above, is all over the WWW.