This is actually a case of using the "singular they" pronoun. Various people have been debating the correctness of using this for a very very long time, but it is generally considered fairly acceptable nowadays (even if it is not necessarily taught as being correct by some grammarians).
One thing that is confusing about "singular they" is that despite the fact that you are referring to a singular individual, because you are using the pronoun "they", the verb is still conjugated based on the conjugation rules for a subject of "they" (sometimes referred to as third-person plural conjugation), so you essentially use a plural-sounding verb conjugation, even for a singular subject.
So actually, what your teacher said is perfectly reasonable, and yes, actually, the sentence in your title is also grammatically correct, even though it seems a bit strange to mix "gets" and "get" for the same conceptual subject. This is because the conjugation of the verb is actually not technically based on the conceptual subject, but is actually based on which specific word is used for the subject of that clause. In the first case, the word is "someone" which is conjugated with third-person-singular, but in the second case, it is "they", which is conjugated with third-person-plural, even though it's actually referring to the same singular person (of unspecified gender).
Hopefully this made some sense.. This is one of those areas where some of the traditional rules of the language have been bent into a new shape by common modern usage, so some of the terms and explanations can be a bit confusing, too.
