Ok so I checked with my buddy...
He has 2 P51D keys in his account, One that he has registered and one he gave me that I registered, he has NOT used the one he provided me.
When we both launch MP and click on the "Register Account" tab, the license key that he provided me shows on my screen, and the one he bought for himself shows on his screen.
I tried to change my password, and we both got an email notification, mine had my login details and his had his own login details.