Hello and welcome to e health forum.
It is understandable that your friend is concerned that her husband is returned back to his old ways of drinking alcohol even after a liver transplantation.
The effects of continued alcohol ingestion after a liver transplantation can cause progressive liver dysfunction, which can be detected by blood tests (liver function tests), elevated liver enzymes.
Clinical presentation, might be delayed but would present with loss of appetite, fatigue, nausea, vomiting, jaundice, etc.
These are some of the clinical signs and tests that might be helpful to know alcohol abuse.