I would say $20 a hen. My neighbor's dog did the same thing and they paid...willingly. Yeah, it's great to forgive and forget but at the same time, if my dog did the damage, I'd pay and then some. If I spilled juice on a rug, I'd want to pay for cleaning, If my kids broke a window with a baseball......I'd tell them "it's consequences"....
That's what I was thinking-ask her to put the money towards a fence that prevents you from having to go through this again. If it were me, I would offer to help (hoping she did't take me up on it, but the safety of your chicken familly is the most important). I'm sorry this happened to you-good luck!