Oh boy.... well I hate to say this but for what your describing the only thing best for your parents cat is to see a Vet as soon as possible.
about the only thing you could really do at home is to see about keeping the wound as clean as possible, a clean dish rag ,warm water and a very mild soap like ivory soap, or if you have some hydrogen peroxide at home that would help.
I'm sorry I could not of been more help, it's just that only the vet will beable to help your parents cat right now.
Take care & good luck,I hope everything will be fine.
Regards,Barry
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