I think any kind of physical damage would come a lot more from chronic, heavy drinking than benzos. I'm talking mainly about hepatic (liver) damage like cirhosiss (scarring of the liver) and the like. How often and how much does he drink, and for how long? Is he a long time, chronic alcoholic? The withdrawls from benzos and alcohol can both land you in the hospital (both are usually treated with a benzo ween, lorazepam/Ativan seems to be the most popular choice in hospitals in the U.S.). Generally, alcohol, benzo, and barbiturate WDs are the ones they will hospitalize you for. Go to an ER in opioid WDs and they will send you home (rather cruel in my book, as well as dangerous. They say opioid WDs aren't life threatening. Ask Jerry Garcia about that. Oh, that's right, you can't because he died from NOT doing heroin by checking into a lock down, sweat it out rehab instead of a medicated, pamper you rehab which he could easily afford, while in poor health from diabetes, heart disease, various health complications like being overweight and getting old, etc...).
Keif' Richards and the others are right in saying that only doctors can tell, with a battery of tests, what, if any, permanent damage has been done. It sounds kinda weird that he required hospitalization for Xanax and booze. Did he just pass out and his sister thought he was in danger, or was the guy turning blue?