Cervical injury resulting in surgical fusion... Is SS still possible?

Hello.
I have been barbell training with the Starting Strength method over the last 12 weeks and enjoying the results.
I have a friend who is recovering from a mountain bike accident that result in spinal fractures. He now has cervical 1-4 fused.
He was inquiring about my strength training.

Does his cervical fusion prevent him from back squatting? Would he be limited to some other variation? Or maybe a different bar or something?

Thanks in advance.

I have a fusion at C6-7. Training has been fine for 20 years. He doesn't need to be looking around anyway.