If he gave you the paperwork that came with the safe, there is a sticker on every SENTRY safe manual with the master code.
I have a safe like that at my office and if I ever forgot the code, I can revert to the sticker on the instruction book to open it.
Mine is a big white sticker with a 0A3807 at the top, followed under that by BC727211* and then under that are two more numbers, one of which is the 5 digit master code.
If he can remember any numbers, start guessing from there. Really there is no easy way to do this. They will not put a master code on safes, that wouldn’t make them very safe. Or contact Sentry and provide proof of ownership.
put op a post on craigslist for cracking it, or thers a place on main street thats good, also if u drill the keypad and get it open, u might be able to buy a new key pad so its totaly not a waist