First off, the friend’s parents are jerks. Just wanted to get that out there in case there were any questions :).
As far as minimum age, it depends on a LOT. At age 7 I would not let a kid have unfettered access to a computer with internet connections, and would require that when he does have internet time, it is someplace public in the house - e.g. not in his room.
Hell, my kids are 17 and 14 and their computer access is solely in a room we pass by all the time. They also only get an hour a day on weekdays, and alternate hours on the weekends. This is because they have proven that they will be online 24/7, to the exclusion of food, sleep, hygiene, school, etc. if we let them.
Not that limiting their computer time has fixed the other issues… they still will find things that are more important than food, sleep, hygiene, and school.
As far as online time: as I said, I support strongly restricting that and making sure it’s somewhere visible. I’d also put parental controls on the sort of sites he can go to, make sure the antivirus is up to date, and counsel him on “Safe Hex”. And periodically review the sites he’s visited in the browser history or whatever. You might want to set him up with a non-administrator user on the computer and lock down the admin user so he can’t get into as much trouble downloading any nasties.
We got bitten badly by something my daughter went to last summer - all of a sudden, the desktop started getting a lot of virus warnings, all coming from her user. We still don’t know where it came from, it took a lot of hours to clean up. This was all despite her having non-admin rights (that didn’t prevent the infection but limited the damage) and some parental controls over websites.
Anyway, beyond the online issues, other use of the computer (games, reading, writing etc.) is your own business. If you feel he’s responsible enough to take care of it and use it appropriately, go for it.