Eh. I mean, if the wood elves can become friends/on good terms with Frederick, I don't see why Bretonnia would feel that differently. Granted, it'd still take saving one of their provinces and its leader twice, but, I mean, it's theoretically possible.
...god, Frederick's accomplishments just look surreal from the outside. Like, it makes sense that Magnus the Pious, a noble of the Empire, reunited the Empire and led it to victory against a Chaos invasion. It makes a lot less sense for some drunk mad lad from Ostland to save the dwarf High King by jumping down the gullet of a Colossal Squig and cutting a hole to exit through...immediately after saving a province of the Wood Elves from a surprise Beastmen invasion...and then getting an offer of a grand boon from the Everqueen herself.
I feel like Frederick's just learned to roll with the craziness by now and is just unfazed by it...kind of like some of the characters in Dragon Ball Z Abridged.