Mega stone trick

Ive seen similar posts a few times but none for this exact scenario, if i have say a Raichu and another ally pokemon sent out at start of battle, raichu is holding a generic item let's say sitrus berry. The ally pokemon is holding raichus mega stone, can the ally pokemon trick the mega stone onto the raichu? At first is was curious about stone switching to pick your preferred mega for the enemy, I know you cant take mega stones away from their matching owner pokemon, but can an ally transfer a stone that doesn't belong to them onto the correct mon?