How to decide when someone is at fault when colliding during overtaking?

Sorry but your partner is at fault. Anytime that you pull out of a parkin, you are only allowed to do it if it is safe and you do not obstruct any traffic. If your partner was hit, then it was not safe to pull out and he will be found at fault. Having the right of way is never a defense, as it is always dependant on if it is safe to proceed.