If she is seriously injured you might want to consider retrieving her. Next see if there is rehabber near you or a vet who treats wildlife (not likely on the second but it can't hurt). As far as treat the shell, the best thing to do is keep the area clean. If you can flush it with sterile saline solution that would be good and be sure to get all any debris out of it. Hopefully no internal damage was done. DO NOT attempt to patch the shell up in anyway. That is a sure fire way to trap bacteria inside the body cavity and make for some serious infection problems.

Seriously though, it is best to try and find a rehabber in your area who may have access to resources which could help this animal.

Good Luck.