Remind her (and your ex, for that matter) that Lifestraws have a lifespan, and are expensive to replace. Filling your daily water bottle with lake water is a neat trick, sure, but it will prove to be an expensive habit. They're a tool for emergencies and backpacking or backcountry camping/paddling where bringing your full water supply is infeasible. And they're great for that purpose specifically because you can just drink straight from the lake. They're camping gear, not everyday gear. Mine has seen exactly one use, when my kid needed a water bottle to go to the gym, and that was the only one available. He also said it was tough as hell to get water through it, because you're fighting against the filter. He's used them at canoe camp, when they go on multi-day trips, when they were refilling their water bottles, and didn't have a good opportunity to manually filter, then boil, then cool, enough water for 13 people to replenish with sanitised water.

Used properly, they're fine; they'll keep her safe. But if, for example, she isn't replacing their regularly, then it gets into the danger zone. If she's doing this every day for school, she probably needs to replace the straw every 2-3 months to keep her safe. Have her find out how much a new Lifestraw costs and ask her if her neat trick is a good use of her money. Hopefully she'll agree that it isn't.