I have been aligning cars for 15+ years for what that's worth and
> The only issue I see that would arise from replacing 1 tire is a lead
> condition to one side or the other....The difference in tread depth
> means a difference in the rolling resistance and circumference
> between the tires...Which creates the pull........What I would
> recommend is putting the mixed set on the rear of the car and the
> matched set on the front..This SHOULD resolve any lead condition that
> may arise ...By the time rotation time comes around you may or may
> not experience a problem with the mixed set up front, and that could
> be addressed then by swapping tires around...
> Aligning the car is also not an issue....The car can be aligned with a
> difference in thread depth...Also remember even brand new tires can
> have a tendency to lead to one side or the other even with perfect
> offsets of Camber and Caster and the same tread depth....I can't
> begin to tell you how many times a pull has been fixed by simply
> swapping front tires from left to right....
>
He could just bite the bullet and replace the second tyre to maintain a balanced set.
Huw