That's actually not that bad, and certainly no reason to be putting her to sleep when all she likely needs is a course of antibiotics to clear it up.
Though with this being a reoccurring issue for her, I would take steps to help prevent it in the future. It could be she is having bladder irritation from excess calcium build up in the bladder(which can lead to a UTI developing), and if so reducing foods high in calcium in her diet should help. Also feeding plenty of low calcium veg like green/red leaf lettuce, etc can also sometimes help because of the high fluid content. Just make sure to introduce gradually and one veggie at a time if it's a new veggie for her. And if she currently drinks from a bottle, change her to a water dish as rabbits drink better from them.
http://www.bio.miami.edu/hare/urinary.html
https://rabbit.org/bladder-stones-and-bladder-sludge-in-rabbits/
https://rabbit.org/bladder-disease-and-bladder-stones-in-the-rabbit/
https://rabbit.org/lowering-blood-calcium/
There's this supplement with cranberry that some say has helped their rabbits with bladder sludge issues. Though it would depend on if your bun will actually eat them, which some rabbits seem to not want to. There is also one by oxbow, but I don't like the ingredient list on those as much because of the added grain. So this might be something worth trying. Or you could try and figure out your own urinary support supplementation.
https://www.amazon.com/dp/B016N0SONA/?tag=skimlinks_replacement-20