Inherited tenants - OK to do different rents when lease comes up?

[Ally Kumar]

I recently acquired a triplex, each unit is $200 below market rent. Tenant A is month to month, is a disabled elderly woman on low fixed income. Her son who doesn't live in the area told me it would be very difficult to move her, there is a rental shortage in the area and she can't afford market rent. At son's request, I verbally agreed to do $50 discount from market rent and do month to month lease as she may not survive a year as per her son. Now tenant 2 has lease ending in Sept and wants to renew at below market rent and is arguing that I am giving Tenant A a discount and not her (she talked to Tenant A). This is the first time, I am handling this kind of situation. Am I obligated to charge all tenants the same rent? I was being considerate of Tenant A's situation but don't want to under rent the other two units as it was purchased as a value add deal based on current market rents. I know I do have the option of not offering other two tenants a new lease and get my own tenants. In general, is it a bad idea to work with tenants based on their particular situation? Can I be accused of discrimination? I didn't find anything specific in FL landlord statues regarding this. Any suggestions/ideas/comments? 

Thanks to all the people who contribute their valuable advice to newer BPers like me. Much appreciated.