Tell her it's medicine for cycling. Medicine isn't supposed to taste good. Is it?
I assume this is with plain water that she is complaining of the taste? If anything else that has a sweet taste already to it, then I've always had to dilute it when riding. Partly because it seemed like cycling hard magnified the taste of anything. And if it had even the hint of a syrupy taste when not riding, then it was almost undrinkable for me when riding.
New bike bottles frequently do have a taste to them. I'll use 24 oz drink bottles from the convenience store and just put my own mix in them. Some fit my bottle cages very well.
Net take-a-way is that for some of us, taste seems to change while exercising.