"I drink her water."
Translation:Je bois son eau.
In front of a word (adjective or noun) starting with a vowel, the feminine adjective "sa" becomes "son" to ease the pronounciation (and avoid the hiatus A-O here).
On top of it, you are probably aware that possessive adjectives in French agree in gender and number with the thing possessed, not with the owner.
So, you say: "il boit son vin" (the water could be his or someone else's); "elle boit son vin" (same comment) "il boit son eau" (same comment) "elle boit son eau" (same comment).
Elle means she,and sa/son means her.
Elle a un chat
(she has a cat)
Son chat est noir
(her cat is black)