The Edelrid Ultralight is a really common helmet to see in outdoor centre kit stores. I have seen a fair few with rusted rivets like yours.
Any one of these helmets left damp, or not somewhere that it will dry out naturally, might develop this issue. Same as leaving alloy krabs in the bottom of a damp tackle bag, they soon go white and fluffy. I'm not saying this helmet has not been stored well by yourself, but you might want to think about storage if that might be part of the problem as a way of extending the life of this and future kit.
I'm sure you can do some light scrubbing with a scourer to remove the rust. If it is surface tarnish, then you're probably fine to continue using it. If the rust, when cleaned off, leaves pits in the rivet surface, then the structure of the rivet is compromised and I'd retire the helmet. It is however your call on it's condition as you are the one with the helmet to inspect.
I suspect the CS lids were assembled with brass rivets and the Edelrid with steel (painted).
Realistically, the only way you will get a true answer about the effect of the rust on the helmet, and any action you can take to prolong its life, is by going direct to Edelrid, with photos and history of use. I suspect they will tell you to replace it.