I would stay away from using glue in repairing any helmet. The glue may react and cause damage that you can't see. I'd use a double sided tape. I've done this on mine where I broke off the small bubble vent on the top. Works like a charm .
If they don't fix under warranty if any, use a 2 part epoxy- one tube is glue, the other hardener, mix with a toothpick and apply with a toothpick. If you need, I have some from previous projects. I wouldn't worry about causing any damage either, cosmetics are the only concern if you're messy putting it back on. Manufactures build hype around tampering with helmets for liability reasons only; there's no real practical concern of harm with what you're doing.
EE-POX-EE is da way to go.
I have custom painted (and repaired custom scratches due to custom get-offs.) many helmets. the problem manufacturers worry about is not with the shell deteriorating but with the shock absorbing lining(styrofoam). try putting Gas in a styrofoam cup and see how long the cup lasts. Mineral bases ie. solvent,gasoline, thinners etc. will destroy your lid befor you can say "what the hell happend to my helmet?".
you don't need a lot; the vapor will do just fine, to put your brain bucket on the shelf for good.
Remember that the next time your buddy parks his bike and puts his helmet on his gas tank.
Like Lugan said, use a tooth pick and a small amount.
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