My Lady has been on Phenobarbital for seizures for nine years. Phenobarbital is known for damaging the liver.
Has your vet been monitoring Roxy all along? That is critical for dogs on Phenobarbital:
PHENOBARBITAL DOSING AND PRECAUTIONS
When it reaches a toxic level, it is necessary to lower the dose or even stop the pheno. Other drugs that are not metabolized through the liver like Keppra can be added:
Reducing Phenobarbital
Lady's ALT values went up to 300 a few years ago (normal is under 100). She was just rechecked a few weeks ago and her ALT was 79 - back to normal! The liver is a wonderful organ that is capable of regenerating itself. With Lady, I switched to a homecooked diet for dogs with liver disease and added Denosyl. She has been getting Marin (milk thistle) for the entire nine years she has been getting pheno to protect her liver.
Dr. Dodd's liver cleansing diet is an excellent one:
Liver Cleansing Diet and Milk Thistle
I would definitely try to see if you can reverse the damage before putting her to sleep. You may want to consult an internist about her medication if your vet isn't familiar with alternatives to pheno or isn't regularly monitoring Roxy's levels.