Fiber carbs “don’t count”?
I think that depends on the diet.
Do you happen to know Elisheva and know what diet she’s on?
Technically bread made with whole wheat flour still has a lot of carbs, they’re just a different type of carbs that are metabolized differently.
The best way to have your kezayis and eat it too is to look at that other high carb culprit – Sugar!
Most homemade challos are full of sugar. Eliminating the sugar will definitely change the taste and most likely the texture as well, but substantially reduce the carbs. There is an obligation to consume a kezayis of Hamotzi – meaning wheat flour based bread; you can dispense with the sugar and create no problem with having Seudas Shabbos.
If there are medical issues involved, and getting rid of the sugar isn’t adequate, then I’m sure a Rav can help determine if Elisheva can dispense with using the grain-based flour as well.
You might have to consult with the Rebbetzin for the recipe though.