They did something very wrong without doubt in my mind. your bikes 2 years old and the spark plug was so tight it sheared of, iv got a 1983 honda vf750 magna and it was in a shed untouched for over 25 years, iv got no mechanical training other than I know what a plug spanner is for but I managed with abit of patience,wd40 and abit of diesel to remove 4 plugs that had been untouched for 25 years plus.
to my mind a trained mechanic should be able to coax a stuck plug out with out it breaking, even more so one that's only been in a while.
That said convincing the garage that its there cock up and up to them to fix it may prove harder than juggling jelly.
