Just a wipe to clean it, should evaporate in seconds. I don’t like getting solvents near polystyrene helmets but a little wipe on the shell, what could go wrong? Don’t think it will mar the finish but maybe try a tiny bit first where you are going to stick the mount anyway
I assume you are talking about cleaning a patch to stick the go pro mount and not the whole helmet? I’d just use water on the rest of the helmet.