Learn English – Word used to denote a glass of drunk water

In agreement with talrnu’s comment, I think “window shopper” is in fact appropriate here. In usage (which might be better represented on Urban Dictionary and Yahoo Answers), this term is commonly employed to refer to people who walk around both inside and outside of shops, and is used to differentiate such people from those who have more of an inclination to make a purchase.

From Urban Dictionary:

1. Someone who looks at stuff they can’t buy.

2. When one visits a store or mall to admire goods rather than to purchase them.

From Yahoo Answers:

It implies [. . .] someone who is not serious (a shopper who has no real intention of buying [. . .])

“Browser” does not sound right to my ear as it seems like an awkward nounification. Most people will think of something they use to view websites. I have not heard the word used this way. It would probably require explanation, which would defeat the purpose of having a single word for it.

That said, I have heard people say that they are “just browsing” to indicate that they are not actively pursuing a purchase at that moment. For your case, I could certainly see someone saying “he’s just browsing.” As far as saying “he’s not a customer”, you have another problem.

Anyone visiting the store is, potentially speaking, a customer. so it’s not ideal to say “he’s not a customer” unless you are pointing to an employee, security guard, pomeranian, etc. You might be clearer if you qualify the word “customer” before you establish this comparison, for example: “he’s not a serious customer, he’s _____________.”

Lastly, another term I know for such a person is to describe them as a looky-loo.

looky-loo

(also lookie-loo)
NOUN
^INFORMAL

1.1 A person who seems interested in making a purchase, but whose actual intention is only to browse:

a treat for all the North Shore’s looky-loos: the popular Spring Designer Kitchen Tour

A good business broker will separate the real buyers from the looky-loos, bring in more qualified prospects, and usually can garner a better price for the business.

_{Source: Definition of looky-loo in Oxford Dictionaries}

As a native speaker, I wonder: is the problem with the water, or with the bottle?

That said, a few expressions come to mind. The first one is the most direct:

You might not want to drink from that bottle; Ann already had her mouth on it.

The second is more of a euphemism than a scientific fact:

Don't drink from that bottle – it has germs.

(We don't know for a fact that the water has any contaminants, but the word germs is often used to refer to unseen microorganisms that could spread disease, particularly in informal speech.)

Because the concern is mainly with someone else drinking from the bottle, you could also say:

Be careful! That may have someone's backwash in it.

TFD labels this definition of backwash as "informal", while the Urban Dictionary says:

Backwash is often created inadvertently or unintentionally when liquid escapes the mouth during the process of drinking .. When multiple people drink from the same container, there will usually be some amount of backwash put back into the container.

As a footnote, the adjective for drinkable water is potable, though anyone who would deem bottled water as non-potable simply because someone else drank from the bottle is probably using extreme hyperbole, or else is an overly sensative germaphobe.