"¿Por qué no habla con su madre?"
It would need to be "hablan" for "they" to be an option. Él/ella/usted are the options. So there are at least two other possible translations "Why does he not speak with his mother?" and "Why do you not speak with your mother?" Looking up "singular they" https://en.wikipedia.org/wiki/Singular_they it would seem to be possible to use "they" here. That is if "singular their" also exists. From what I read they are both common in speech but should be avoided in formal writing.