Law prevents more than 15% of social security wages being garnished for student loans, so it would be good to understand how his living situation is getting priority over other bills. His wages could be garnished for the foreseeable future and this would be a long term reduction if they are not able to work. If the tenant's only plan to make the numbers work is to get a reduction from you - then that is a problem.
The tenant needs to get familiar with this: https://studentaid.ed.gov/sa/repay-loans/default/get-out#loan-rehab
Other things to consider:
After what period of time does it make more sense to replace this person with a tenant paying market rents compared to your reduced rate? If he owns his home has anyone played with a Reverse Cash program (e.g. Park Owner gets credits to purchase the tenant's home after X amount of time)? Does tenant have any other assets you can use to compensate / collateralize for the reduction (e.g. car, collectables, etc)? Can tenant start doing some work around the property to 'earn' the reduction - think mowing common areas, have skills as a tradesman?