There isn’t a good parental control software, free or otherwise. Even when they’re do manage to block ‘inappropriate’ sites, they almost always also block ‘appropriate’ sites that a researching teenager would want to visit legitamately.
And even when they’re in place, they’re fairly easy to get around by routing your traffic through open proxies. If China can’t filter all the traffic coming in on their trunk lines to effectively block access to sites that it’s government finds inappropriate, your friends aren’t going to be able to filter the traffic going into their son’s computer on no budget.
If they can’t trust their kid to handle anything he finds, and they must have more control, they’ll have better luck with monitoring software, which logs everything the kid does, and then they can review the log and discuss any ‘inappropriate’ activity.