Nonetheless that wasn't a wise move dallantan!, you should have got your mate to bring a portable mifi device with him so his accounts would always be separate from yours, those devices are excellent to use, for keeping things separate. Particularly good when anyone else in the house goes online as well.
I've got a 3 one dallantan, they come in various shapes and usually small sizes, just connect up to it via your computer devices wifi, it will pick up the mifi signal and you or your mate can work away undetected. VPN is great as well and can be acquired via this forums marketplace. The 3 mifi cost about £50 if I remember right, can top up online when the time comes which is convenient as well.
An IP address being used once is not enough to cause a definite link.
As i am sure people who live in your area using the same ISP will incur the same IP you get at least once in a lifetime.
(Doesnt always happen, but it can happen)
But if he used your IP & same PC/User account, that is a different story.
What if a friend stays? 
