I agree with
@Piggies&buns , 4 of our piggies came to us as youngsters (youngest 9 weeks old) and we fed them a normal balanced adult diet, grain free pellets, unlimited hay, mostly low calcium veg like peppers and cucumber and coriander. We might feed a higher calcium veg twice a week but definitely no more and in limited amounts (like 2 or 3 stems or parsley, or 6 spinach leaves, or 1 piece of baby broccolli). They have all become healthy adults no problem, even our skinny pig and there are many online myths about skinny pigs having specialist diets!
This is what I have done with Dot and Bernie, and I have also lost a Guinea pig to 3 bladder stones, and back then I fed too much Parsley, kale, spinach, although I was feeding the same diet to both boars for years, one died age 6 from 3 stones, his brother ( same litter) died age 7, not once did he have bladder problems and he always peed white stains out but never gritty.
I believe it depends on the Guinea pig and the diet.
I stick to this daily
Grain free science selective ( 1 tbl spoon, per pig per day)
Daily veg - 1 sprig coriandor, red and green bell Pepper slices
Cucumber 1 ring each, 1 ring of parsnip each and 1 chuck of celery each.
Grass daily when it is nice weather out to pick it.
Filtered tap water trough a wilko filter jug.
I refuse to say so far so good, because there is a beast up there, somewhere in the sky that's sees life is going well and will give one of my pigs a problem.
I add in kale twice a week, and dill, and some raddichio