I believe so but here's why. To paint it you would have to take it out of water to paint, no while I know they can be out of water for a while I feel it shouldn't be done on purpose. Secondly, you would have to let the paint dry which would be an even longer time out of water, which is where I'm thinking that if you need to paint to dry then the snail might dry out also, which would kill it. Lastly, it would be near those fumes while your painting which could ruin its health.
How about a nice medium, let her paint a few empty shells, give them a week or so to dry out completely, then buy a small hermit crab and hope it moves into one of the shells
Edit: if you do the hermit crab option, don't paint the inside of the shell and uou will also want to do a lot of research into paint for the shells. You will want to find one that won't leak toxins, won't fall apart and is reef safe.