Deleted

If they send him a link and he clicked on it, sure why not.

Trust noone.

What could very well have happened is, they told him to install TS, but gave him a fake installer, or one with a trojan bundled in it.

If you are using a legit teamspeak software (downloaded from the ts website, its not a trojan, maybe his computer had some kind of vulnerability, so when you enter to someone's server they could attack you directly (because they have your IP), but again, if you have a router and some kind of firewall its kinda difficult....

Possibility 1:

• He went on the ts
• They gave him a link to whatever (funny picture or what not) that had a virus

2:

• He downloaded files from their filebrowser on teamspeak
• He ran that file and got infected

3:

• They hacked him
• Uploaded the virus

4: (not sure if it even would work like this 100%)

• They hid an virus file inside an image
• He opened the infected file
• Virus installed on the pc

That is to assume they didnt tempt him into trying a csgo hack of course :P