Horse's water sudsing up after drinking

My friend is boarding her horse at her family's property and one of their horses is sick. I was wondering if any of you have delt with something like this, I would really appreciate your experience and help.
A few days ago the horse stopped eating and his gums were dark in color. Also when he would drink his water the water started getting sudsy, we changed the water and it continued to suds up. We even went as far to get a new container, again it suds up after he drank from it. They took him to the vet and she didn't know what was happening with him. He was started on antibiotics but he doesn't seem to be getting any better or any worse.

Have you ever encountered this problem? We're becoming worried about the other horses on the property, none of them are showing signs of being sick. We have no idea if it's something that is on the property or if someone might have intentionally done this to him.
Thank you for your help.

Our horses who "wash" their mouths in water after eating grain.. they have suds in buckets. I don't knwo if that's the issue.

The horses at the barn where I originally started riding constantly had this happen. BO, vet tech boarder, and vet said it was just because of the horses' saliva. Nothing wrong, just the fact that when they drank, their saliva mixed with the water, creating the "sudsy" effect.
I, personally, wouldn't worry too much about it. I know my old barn used all well-water, so it could just be that the well water and saliva did the "sudsy" effect and the normal tap/city water has stuff in it so that it doesn't create the sudsy effect. I can't say that for sure, but I can only speculate with possibilities from the experience I've had.
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Has he started eating again now? The vet had no opinion on the lack of eating? Did (s)he check the horse's teeth? Gut sounds? Is he passing manure? Is he urinating?

I wouldn't worry about the "sudsy" water, but I would hugely worry about the not eating.

As others said, it could be saliva. It could just as well be the water...but as you said all of the other horses are fine my best guess is it is just the horses saliva.

Why would well water do this????

It has far less additives in it than city water.

None with out a water conditioner.

It's just the bubbles forming from the horse's action in the water with the saliva and possibly food particles mixing in. Remember there were always the kids that you'd never share your pop with because they'd spit back in the bottle??? Ya, that.

It wouldn't hurt to get the water tested. But my guess would be from the saliva.
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I wouldnt know why well water would do it. Never know unless you get it tested :)

A properly constructed water well is one of the safest ways to get water.

BTW, my horses also "slushed" in their water.