[morethanpretty]

I hink you might need to examine why said person is invisible. If it's due to their skin causing a light emitting organs on the skin, than technically the invisibility will only be skin deep, so any waste would not have the benefit of these organs. In that case it would be a simple matter of looking in a mirror to make sure you're clean. If the invisibility is something system wide where all of your organs/bones/ect are literally see through, then that's a tougher question. We still need to know the mechanism thy causes one to be invisible and if foreign matter in the body is also rendered invisible.