In my opinion she's drinking too much if she's peeing that often. Some people sweat more than others and need to drink more and they aren't necessarily working harder than those who don't sweat as much. Her superstar BF probably needs to drink a lot more because he either works a lot harder and/or his particular body just needs more hydration.

Also, some people just simply need to pee a lot more than others, whether riding a bike or not. I am sadly one of those people! Before my first longish ride with DH on the tandem I said, "And there's potty stops, right?!" However, I have found that the more I drink on a ride, the more I'm certainly going to need to stop to pee. Or hold it! I don't sweat as much as some people, so I think my body makes me eliminate it differently. I think I really just don't need as much to drink.

Speaking of her superstar BF, my superstar DH just did an amazing century coming in 4th out of over 300 with 10,400 feet in elevation. His most impressive ride yet. He planned not to stop even once, but had to when the guy he had teamed up with simply had to stop briefly to refill his bottles. Dh was going for speed and adjusted his eating and drinking to accomodate that. He stuffed a bunch of bananas and nutrition bars in his jersey, and figured out how to stay hydrated without having to stop. It can be done! And he's serious about getting the energy you need from food and liquids.

Everyone has different things that seem to work for them. If she seems to think this is the way things have to be done and won't bend, then she's obviously not a suitable riding partner. But, as others have suggested, you might gently suggest less and shorter breaks and, over time, you guys may come to a middle ground you can both be happy with. She may come to realize she can get by with less potty stops and/or carrying her "fuel" in her jersey to either eat while riding, or something that she can eat wthin a minute or two when you do stop.