It's a bit hard, since the A/C is one of those old ones attached to windows after all, but after fixing some broken parts, replacing some other parts too broken to fix and eventually cleaning the entire thing. You were able to fix the A/C. 

"Hold on I'll check with my sister, HEY!!" she yelled at the inside of her home, "Is the A/C working yet?!"

"Ya! It's good."

"Cool. A/C's good," She looks back at you. "Well I'll be damned, somebody was able to fix the damn thing. Just hope it last long." She sighs.

You suggest buying a new A/C to her. 

"I could, it ain't cheap though..." She scratched her head.

You offer to pay it. With the reason of just trying to be nice to neighbors 

"Seriously? Cool!" Despite the good news, she doesn't smile, still keeping that expressionless face. "Hey, thanks for helping with the A/C man, anymore nagging from them and might snap."

Again, you told her you were just trying to help. 

"Huh, well I don't feel really good not repaying you or something..."

Well... There is something she could do to return the favor.