I don't think it'd be tire pressure unless they were really low - maybe balancing> did your friend get them from the same place or have the same shop mount/balance?
I think balancing because the only time I experienced what you're saying is when I had new tires mounted on my bike - only above 80 did I feel the front end start to vibrate a little. They re balanced and that was all it needed.
I think balancing because the only time I experienced what you're saying is when I had new tires mounted on my bike - only above 80 did I feel the front end start to vibrate a little. They re balanced and that was all it needed.