As far as I know, chickens don't get sunburn. (I learned this from another member who had Naked Neck (chickens) who were in a lot of sun but never got sunburned.) The red skin is something else which normally happens in bald spots, with or without sun exposure. There's nothing that is needed to be done, though I'd make sure that her bedding is clean and that she's not rubbing her belly too much.
On a side note, the skin looks a little loose (stretched, hanging). I'd have a higher concern over that than anything. On all my hens, including the ones who have bald bellies, their skin is tight against their body and isn't hanging down like that.