Addressing someone.

[Bromad]

So it's about something that just happened at work. A Colleague from a different office asked me for information about a client of ours and I answered referring to the client as "They". After my reply providing the required information, they froze and just sent me a "?". Even my manager looked at me and said why didn't you say "He" referring to the client. I Was shocked and had the words "wtf" just going through my mind for a few minutes, which has lead me to now ask this question here. Note that my office or the one my colleague is from are not located in a native English speaking country. So this made me doubt my language skill and would want to ask someone if it's actually grammatically correct to address a client the way I did.

[Alan]

It is common practice in spoken English to refer to a 3rd person singular as 'they' when his/her gender is either unknown or unimportant. More formally, however, '(s)he' is usually preferred.