Don’t tell me they/them/their cannot be used in singular form. Don’t tell me they/them/their is “grammatically incorrect”. They/them/their does not have to be plural and can be used singular.  So if someone directly tells you that they prefer these pronouns rather than your typical day to day “she/her” or “he/him” then don’t throw a fit or make it hard on them by trying to throw out the “that’s not grammatically correct” excuse. Yes, I admit, being myself as someone who prefers general neutral pronouns, it can be challenging to get the hang of they/them/their in singular usage, but that doesn’t excuse you from respecting someone’s identity. Don’t be an asshole, respect people’s pronouns and apologize if you slip up. It’s that easy, and yes, it’s required of you as a decent person.