Your friend has a number of options. Next time he gets disconnected, have him try one of the following things:
1) If he's running a wired connection a.k.a. an ethernet cable attached to his PS3, have him unplug it to reset the internet, then plug it back in
2) If he's running a wireless connection a.k.a. a wireless router, have him turn it off for 30 seconds them turn it on to reset the internet
3) Have him clear his system cache in GT6 by hitting the 'Start' button in the main menu, going to Network, and clicking on Clear System Cache
4) Restart his PS3
5) Turn off any other systems such as a computer or phone to speed up the connection
6) Set Race Quality to Normal or High instead of Very High. Doing so takes away stress that his internet might be having
If none of these options work, then either his NAT type is incompatible with yours or he needs a faster connection. Have him do an Internet Connection Test. If his NAT Type is Type 3, he's most likely going to have problems. Also his download speed should be at least 10 or above, and his download higher than 2. Good luck! 👍