It's not 'rebuilt', it's 'refurbished', but there isn't a specific definition of what that means.
It could be rebuilt, or just degreased at a coin-op car wash...
Even rebuilt isn't a specific term. Could be rings, seals, bearings, resurfaced and cylinders honed, or could just be new gaskets so it doesn't leak oil, for a while.
I wouldn't touch it unless he had receipt for parts and labor. And I would call or visit the shop that did it.
FYI; I use to rebuild small block chevy V8s. I have one in my garage that I blue printed, balanced and had the heads ported.