If she wants color, then I've had good luck with Colorado and Jack Wood water lilies. They're fairly tolerant of water fluctuations, slow spreading, but a prolonged water drop will get them.
Arrowhead's a good plant, but again, a fairly constant water level is important. Some of the sage's and reeds are nice too, but they can spread quickly around the shoreline. The reeds have been the most drought tolerant plants at my place.
Be careful if you go shopping in person or online, and do research before you buy. There's a great water garden shop in Garland TX, and they sell Pennywart. It's as bad an invasive as I have ever experienced, but perhaps that's no problem in a small Koi pond where it can be managed. I offered to bring them an acre of it once, and they declined. I think one trough full was enough for them.
If you start seeing any plants that look like lilies, but you didn't plant them, then take pics and research them quick. Spatterdock and American Lotus are 2 very common wild plants around here, and they're a bear to get rid of.
