He grazed, and then his camels refused to drink.رعى فاقصب.

It is said, "The camel refused to drink," and "The shepherd refused to drink," meaning he neglected to graze his camels, causing them to refuse to drink. The word "grazed" itself doesn't imply negligence or mistreatment, but the word "refused to drink" is used to indicate poor grazing. This is because the camels refused to drink either because their stomachs were empty or full, both of which indicate poor grazing.
This proverb is used to describe someone who doesn't give good advice or doesn't take their responsibilities seriously enough, ultimately ruining the situation.
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The closest English equivalent to this proverb is:
He mismanaged the job.
Meaning: He mismanaged the task until he ruined it.
A very similar proverb and expression is:
He spoiled it through neglect.
Meaning: He ruined it through negligence and lack of care.
Similarly:
A bad workman blames his tools.
Although this focuses more on incompetence.
The closest equivalent to the meaning of "he didn't take good care of it until it went wrong" is:
Neglect ruins everything.
Or:
What is poorly tended soon goes wrong.
This is very close to the image of the Arabic proverb "He grazed until he overgrown his vines."
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يقال: قصب البعير يقصب، إذا امتنع من الشرب، و "أقصب الراعي" إذا فعلت إبله ذلك، أي أساء رعيها فامتنعت من الشرب، وليس في قوله "رعى" ما يدل على الإساءة والتقصير، ولكن استدل بقوله "أقصب" على سوء الرعي، وذلك أن الإبل امتنعت من الشرب إما لخلاء أجوافها وإما لامتلائها، وهما يدلان على إساءة الرعي.
يضرب لمن لا ينصح ولا يبالغ فيما تولى حتى يفسد الأمر.