If you are the general contractor for this job and you are billing the client for the tile work, then you are responsible for any implied warranties. If the homeowner hires "buddy" then "buddy" will have to supply any warranty, if there even is one. As long as you don't touch any part of the tile job or tell "buddy" how to do the prep or install you'll be fine.
Unfortunately scratch/wire is still very prevalent in the Toronto area. And it is still garbage and always will be garbage.
Just ask anyone over at the TTMAC association in Toronto about scratch coat testing.
www.ttmac.com