I’d clean off the rust and then apply a thin coat of grease between the rims.
I think rust builds up from non-use of the scooter. If you ride a lot you’ll need to change the tires/inner tubes couple of years and the rust will not get so bad.
I had the same problem. If you want to do your friend a big favor, I'd recommend sanding off the rust and repainting; otherwise the rust will keep spreading.
I don't have any experience with RTV, but I agree with 2003stellaDave that some kind of grease is probably better than a sealant, as a sealant will make the rims harder to split in the future. I'm a big fan of Fluid Film for keeping moisture out and inhibiting rust. You can safely spray a generous amount between the rims.