That looks painful, poor thing. Maybe some massage followed by epsom salt at 1 Tablespoon/5 gallons, it can help with some swelling problems. Surgery may be needed too but that kind of thing always worries me. Less maybe with a needle stab than cutting. If you have access to a hollow needle like for injections that would be my choice.
If a stab doesn't help either & you have a syringe & needle you could try drawing off the air or fluid inside. You probably will will need someone to hold the snail. You might be able to make a "nest" of a wet paper towel to hold it securely in position. I used to be able to 1 hand that kind of operation when I did it often enough to be fairly confident, I'm pretty sure I couldn't now (wild birds not aquatic animals). Go slow & as gently as you can. If you have a choice a larger guage needle would work better & maybe a 3cc syringe, the bubble looks like it could be more than 1cc, but do the best you can. Don't wait to try something.
The other idea I have is to go to a snail website, maybe they've seen this before. There used to be a few but I don't remember them.
Good luck! & let us know what happens.