Are you sure the neighbour's extension bricks actually tie into your friends house bricks - or have they built a completely separate wall a little distance away from your friend's wall?
If the bricks are tied into your friend's wall (which I'm sure your friend would have to agree to first), I would now class it as a semi & is bound to reflect on the future selling price.
If not, I'm sure it would still be classed as a linked detached, although I'd say it would be a bit too close for comfort!