SPF sunscreens etc don't have much effect, the cause of the bleaching effect of the hair is actually related to it drying out from the heat produced from the sun rather than the light (i.e. UVA/UVB rays).
So a good conditioner is your best bet to help keep the moisture from being sucked out of the hair shaft which leaves it dry, brittle and a rusty orange.
My girl has the same thing, although she is many differen shades of brown anyway so I quite like it. If I had a buckskin or a bay I would probably think differently!