What do you do when donating to a panicked diver who initiates a buoyant ascent?

[Adamastor]

Yes, I'm assuming standard recreational diving equipment.
The OP didn't specify it wasn't the case.

 

[Seaweed Doc]

I watched an instructor punch a guy in the stomach to get an individual to breathe when the guy "locked up"....

Done that. Twice. (Mask remove/replace skill both times.)

 

[Angelo Farina]

Done that. Twice. (Mask remove/replace skill both times.)

I inserted my fingers in the student mouth for forcing exhalation during a panic ascent...
It worked!

 

[chechr]

This might be hard to believe but I dealt with a panicked buddy on the first dive of my life. My wife and I took a class with some of her work friends, including her boss (the OOG diver). At the end of the course, we are in the lovely (sarcasm) St-Lawrence river for the final in water 33ft tests. Once completed, we are all certified and the instructor decides to bring us for a quick tour around the area. We're at the bottom, no more than a minute under way and I feel a tug on my fin. I turn and he's looking at me with huge eyes and no reg in his mouth. I completely forgot about my spare reg, I grabbed his shoulder strap and put my reg in his mouth. Thankfully he didn't bolt, we just slowly ascended and shared the one reg back and forth. He later explained that he couldn't get any air from his reg, but I don't see how that could be.

 

[steinbil]

This might be hard to believe but I dealt with a panicked buddy on the first dive of my life. My wife and I took a class with some of her work friends, including her boss (the OOG diver). At the end of the course, we are in the lovely (sarcasm) St-Lawrence river for the final in water 33ft tests. Once completed, we are all certified and the instructor decides to bring us for a quick tour around the area. We're at the bottom, no more than a minute under way and I feel a tug on my fin. I turn and he's looking at me with huge eyes and no reg in his mouth. I completely forgot about my spare reg, I grabbed his shoulder strap and put my reg in his mouth. Thankfully he didn't bolt, we just slowly ascended and shared the one reg back and forth. He later explained that he couldn't get any air from his reg, but I don't see how that could be.

Probably his valve was not fully open. Did your wife get a raise?