So sorry to read this Rita for you, Barbara the new owner and for your mare Cookie. We've had situations like this - three in fact. One we still have and two have gone on to other homes. The biggest concern from what I was told was trauma to the cervix. The vets had them on two types of antibiotics and on we were swabbing (can't think of the name - the nitrfurizone ?? cream) in their vagina 1-2x a day to prevent scarring.
It is really great you're going to work with your client on this, as technically it's her horse and her responsibility. It's also good for her that she saw and now knows how serious breeding can be.
Best of luck on resolving this, and best wishes for Cookie's full recovery.
Also congratulations on your new filly!