Hang it off a mirror? Carry it with the strap in your teeth? Or, buy a helmet lock, install it somewhere on the bike and strap the helmet on that, preferably in the area of your pillion seat.
If you dropped her off, she should have kept the helmet with her.
If picking her up on a whim, and doing it often, invest in a small tail rack and bungee it to that.
Helmets get banged around dangling from a lock so that's not really a good choice but investing in a small tail rack would be good anyway to carry the odd package now and then.
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