This question already has an answer here:
I was wondering what the general take is on concord with they as a gender-neutral 3rd person singular pronoun. I assume there are no hard and fast rules here yet, seeing that this is a fairly new phenomenon – or am I mistaken here? If there is, indeed, an established rule, please let me know! But if there isn't, what is your view? Do you think grammatical meaning should determine concord (example ), or do you think semantic meaning should determine (example )?
 If someone wants this, they have to let us know
 If someone wants this, they has to let us know
I can't help it –  makes me cringe... even though I realise that there are equally good arguments for either.
What's your view? :)