Sometimes it's unbelievable how stupid some clients are and like your client my nightmare sister is still biting her nails at 58 years of age! I think she is just so used to picking, biting, pecking and putting them in her mouth they stood no chance and neither did your set of nails on your nightmare client.
Keeping them very short as I have done gave my sister 10 days before "shock Horror" she ripped them all off again!!!
They took an hour and a half to apply using forms and there is nothing more annoying than when they have been forced off after all your good work!
I would have repaired the 2 nails of course as 7th Heaven has suggested, but she has not given you the opportunity to right them.
Nail biters have the most horrific amount of cuticle anchored down like super glue to the nail bed so prep, prep and then again to fully remove all the extra tissue off the nail plate. Also there is very little natural nal on which the extension can be anchored onto so I would Keep the extension very short for the first time.
Hopefully the second time you do them there will be far more natural nail to attach the extension on to. Then they can be a bit longer.
Ask her to please give you the opportunity to do them for her again as it it your wish to resolve the issue at no cost to her obviously.
A bit of grovelling sometimes helps, but that's up to you.
Good luck or good riddance to her!