Indoor or outdoor use?
Sounds like he needs something like a
spy cam or nanny cam.
Typically, due to their low profile, they would have to be either hidden inside an air duct or something like that.
There are some cameras that come already hidden inside an common household item like a
smoke detector or plant (for indoor use). For exterior/outdoor, your best bet is probably a wild life cam that you would have to hide in a bush or on a tree.
More expensive
ones come already disguised to blend in with nature and even solar panels to charge themselves.
Some indoor models to have Wi-Fi but then which Wi-Fi would they connect to? If you are piggybacking onto someone else's wireless, they could easily be detected and their cover blown.
For remote monitoring, you might be better off with a camera that supports a large TF/SD card and long battery life and adjust its motion sensitivity according to your needs. Wild Life cameras typically have a Video/Photo option as well so if a series of photos (1-2 secs apart) would suffice, instead of video recording, you could opt for photo mode to prolong battery life as well as the memory card usage. Night vision mode is also available but can suck up battery a little faster.