I'd have my cousin cut it off with my sawzall. I first thought about my smallest chainsaw but then I thought my sawzall would be safer. And yes I would let my cousin cut it off with a chainsaw if I didn't have a sawzall.
First I'd try putting some olive oil on my head and try to slip it out. If that didn't work, I'd try canola oil. I would not try motor oil. But I might try mineral oil. If there was a way to freeze my head while heating the helmet, I'd try that. The contraction of my head and the expansion of the helmet might be enough to get out of it. If all these things failed, I'd use a small hack saw to cut the helmet jaw piece and then take off the helmet.