Are the speed ratings the same for the Defenders and the Premiers on your friends car? Premiers come in T, H, or V rated. Defenders are T, or H. One would really have to pay attention so that T's are not being mixed up with H's or V's, and so forth.
Having mismatched speed ratings on a vehicle is worse than just regular mismatched tread patterns, I would guess.
Is it an AWD vehicle? It's my understanding that an AWD car, all four tires have to be exact diameter, or it can possibly screw up the AWD system components...?
The Premier is ever so slightly shorter in diameter than the Defender: i.e. 25.6" Premier vs. 25.7" Defender in my size of 205/60/16, as listed on Tire Rack. The Defender is only available in T speed for my size, and the Premier only available in H or V ratings. So, if they did the Defender/Premier mix up on my car I would have different diameters and speed ratings.