In what way was your family member hassled?

F9 publishes the dimensions of authorized carry-ons and also publishes its fees:
https://www.flyfrontier.com/travel/t...o/bag-options/

Bag dimensions are binary. Either the bag meets the dimensions your relative agreed to when the ticket was purchase or it does not. The policy may be silly or not, but the time to make that decision is prior to purchasing the ticket.

I am not a fan of F9. But, whatever faults it has, it is clear about its policies and it enforces them.