You know what? I rarely see BonBon at the water bottle, but she must be getting enough from her food. Before she was this started, she was always on the bottle. But to look at her, she's obviously hydrated enough.
If your piggy has never been one to hang around the water bottle, it's hard to gage whether they're getting enough, I think if they're dehydrated, the weight drops off quicker than when they stop eating, as even we as humans are mostly made up of the stuff.
Getting the balance of veg that contains a high enough water content to help encourage a Guinea pig to take in more water, and encourage them to eat more is a fine art, trying to avoid soft stools at the same time. Gripe water, dissipates the gas build up within the pig's stomach, which has been giving the piggy the impression that they are full, when really they're not. The gas in turn makes the stomach produce more acid than it needs, to break down what it thinks is food, irritating the stomach lining, increasing the risk of ulcers. The grip water i've been using works in two ways, it removes the gas bubbles, and neutralises the acid, and it also acts as a anti-inflammatory from the throat to the entire gut system, giving the piggy much needed relief from discomfort. (Can't you see i've been studying it).
With BonBon I think she's been suffering ulcers through stress (back when she lost two pals within weeks of each other) and had been in gradual decline over 17 months, her decline has ceases, and she's now levelled out and starting to improve, but it's been a long haul.