According to a friend on mine, the pronoun HIM in the following sentence is Okay. Whereas, in my understanding it should have been THEM. Please let us know which one is according to the English grammar rule.
"When you get so used to being with someone, you will have no idea what to do without HIM/THEM."
Technically, your friend is correct, but the he/him/his rule for unidentified or unknown gender is not only considered a little antiquated, but it's downright awkward and can lead to misunderstanding. Only people over, say, 40 years old would even know this rule. Many linguists today accept the they/them/their/theirs for the gender-neutral 3rd person singular. Language changes. In formal writing or speeches, however, I suggest rewriting the whole sentence to avoid this difficulty altogether.
