If she is actually allergic to sun then the only solution is to cover up, with clothes and sun screen.
As a first-line treatment, we usually ask the person to avoid the sun
if possible, and if they do go out in the sun to use photo-protective
measures. So in addition to staying in the shade, they should wear a
long-sleeve shirt if possible and use what we call broad-spectrum
sunscreen that has UVB as well as UVA blockers. If someone knows that
they are only sensitive to UVB, it's not as essential that they use
the broad-spectrum sunscreen. We ask people to look for sunscreen that
has SPF 15 or above because the probability of having UVB and fairly
good UVA protection is quite good. People should specifically look for
the word "broad-spectrum" on the label.
If it is heat that causes her hives then the main goal is to wear cooling clothes like light linens.
That is about all there is to it.