Oh, and lavender likes it quite dry.
Is it in a pot? or in your garden? If it is in your garden, just water water water… rinse the urine out of the soil and hope that there is enough drainage so the lavender doesn’t rot. If the cat is spraying the lavender with the gland under it’s tail, I don’t know what you can do about that. Cats start spraying when they feel threatened or stressed about something, some just spray because they’re really territorial.
The whole plant is whithering because it’s taking up the urine from the roots, I’m guessing. You could do a pH test of the soil and perhaps use either baking soda or some sort of acid balancing liquid. A heathy cat has a urine pH of abour 6.5 (slightly on the base side), but when it oxygenates, it would turn more like ammonia and very caustic. It could be killing the lavender from the roots up.
I’m not a cat person. I curse and swear when I’m in my garden and I dig up the neighbour cat’s shit.