Is there any harm in putting hand warmers around your CO2 tank?

If the pressure in the tank gets too high it will blow the burst disk located on the regulator, this is a safety feature on the tanks. The burst disk needs to be replaced once it's blown or the tank won't hold any pressure. The tank itself will not explode. The only other concern is that the preheated co2 might result in higher than expected velocity while firing which can get you hot gun penalties, and the excessive pressure could cause some damage to pressure sensitve guns.

It will not explode. you would need extreme temp to make it explode more then a hand warmer but depending on the weather conditions you could blow the burst disk

Depending on hot it got it could explode!!! We're talking about volume not weight. Warming it will cause the gas to expand exponentially (a multiplier). It's hard to say without more information. Does he really need to do this or is he just being a wuss who is afraid of cold hands? Cold hands or no hands...

Carbon Dioxide is non-flammable... but the pressure in the tank might cause some damage if the gas expands.

theres a possibility

its gonna make a big explosion and burst into flames

the heat may cause an explosion