Is it possible to hack into another poster's computer on a forum?

It is not simple to control a person's PC. Most likley the victim was keylogged and the hacker had his username and pword along with the website. Do not be scared, just don't download untrustful things. For the hacker to have total access of your PC, they would need to install a RAT(Remote Admin Trojan) this cannot be done without a user downloading a virus.