There's no good way to answer your question as such that it would be at all helpful to your friend.
Firstly, the identifier "GB4" applies only to the various replica lines, as far as I know. If he knows the cue as a "GB4" from his purchase of the cue, it's likely a replica.
If it is a Balabushka GB4 replica cue, we would need to know which replica it is (there have been a number of them).
If it is a real Balabushka cue, there are multiple factors we would need to assess to provide a value range.
- Is it from the "Titlist Blank" era, the "Burton Spain" era, or the "Gus Szamboti" era?
- What, if any evidence or documentation of the cue's provenance does your friend have?
- Could it be an early fake/forged version?
Regardless of whether it is a replica or an original Balabushka GB4 cue, we would also need to see clear photos depicting it's condition and key points of interest (e.g. joint, insert, forearm, butt sleeve, etc.).
Condition is everything in pricing used cues.
If there is any possibility that it's a legitimate, original, George Balabushka pool cue, your friend should get the cue in the hands of one of several experts for an George Balabushka cue authentication.
First choice would be Tascarella Custom Cues.