Buying a DOT sticker off Amazon doesn't make a helmet compliant.
If you're talking motorcycle/car insurance payment or any other type of civil liability, it probably wouldn't be non-payment. For example, if someone else caused the collision, they'd still be civilly liable. But it would provide ammo the insurance companies / attorneys would use to try and reduce liability. They would argue that had the rider been wearing a DOT compliant helmet, their injuries would be less severe (whether true or not) and that they should only be liable for partial medical expenses, etc. Why give them that ammo?
It was discussed in another thread within the last 6 months or so, but... I think that if a helmet does not meet DOT standards, that does. It make it inferior. Many ECE standards are higher than DOT/Snell. If the rider can show that the helmet met higher standards, even if not technically DOT certified I think they would be ok in the long run. It would take a competent attorney with an understanding of Euro certs and stuff, because if it’s that serious where there is litigation, the rider is probably in no position to bring forward that information.
If I remember correctly, Suomy helmets are not DOT certified. But..... if they are good enough for the MotoGP grid and meet FIM standards I can’t imagine anyone making a successful argument that the lid is less safe.
EDIT: Correction.... Suomy helmets are DOT, but not Snell certified. The point remains, though.