The drinker is the "effective caster of the effect", true. However, the Share Spells ability clearly specifies:
"any spell (but not any spell-like ability) she casts upon herself"
The drinker of a potion is not "casting a spell" himself, he's receiving a spell effect. The interpretation is contentious to what exactly "casting a spell" implies (and the ability specifically excludes spell-like ability, hinting it is already limited in scope). By a strict interpretation, only spells the master CAST himself are concerned, excluding every magical items.