The 'flag' (a bouncing blue arrow, like the one that appears when you hover over a building on your island, but bouncier) appears for 5 mins, whether a friend clicks on the collectible for you, or you use a Fairy Fire.
That's not the experience I've had. I've let a friend mark some of my collectibles and then let them be for way longer than 5 minutes. Flag was still there. Perhaps BB has changed it?
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