Arlene`s mother got it all wrong.
Trevor is trying to make the world a better place with the "Pay it forward" plan, and Arlene pays it forward to her mom by forgiving her, which is a good deed. But Arlene`s mom is definiteley not committing a good deed by helping a criminal escape justice, when she tried to pay it forward.
The idea was to make the world a better place, but you don`t do that by assisting criminals. She did him a favor yes, but a good deed? No.
Only excuse I can think of is that Arlene didn`t specify to her mom that you have to do something good for someone else, she just told her you gotta do someone else a big favor.