Anyone know how to track keystrokes on a macbook?

My younger brother (who is grounded for his drug use) is sneaking out at night. My mom took away his phone, but he keeps doing it. She wants proof that he’s using facebook to get in contact with his friends before she takes the computer away entirely (he needs it for school work over the summer). She can’t trust anything he says because he’s lied about absolutely everything since Homecoming last year, when he abandoned one of his friends who passed out drunk in a ditch.

Currently, his laptop is broken and he’s been using my old macbook (which is still on leopard osx). I was wondering if there was any way to monitor his activity to get his facebook password and see for ourselves.

Normally my mother and I wouldn’t invade his privacy, but he’s really at risk of hurting himself. He drinks, smokes, gets high off of cough syrup, and recently, my mom found out that he was trying to take my dog’s arthritis pills (an opiate). And that’s just what we know of.

I was wondering if there was any freeware for the mac that we could use, or something. Any ideas?

Edit: Sometimes, he uses the PC downstairs. If there literally is nothing we can do on the mac (without spending a ton of money), any ideas for the desktop?