I recently gameshared with a friend and I no longer want him using my account anymore, so I told him de-activate the account as primary and I activated it as primary instead. He probably still has it as a user on his playstation and I would like to know if he can just select my account to get in and activate it as primary again. Will he be able to make any purchases?
Yes, you should change the password immediately.
Non-Primary accounts are able to spend funds in your wallet. Also be aware that there is a limit of sign-ins per account, to play games or watch videos.