Who knows English grammar?
The top one is, but is sexist. The bottom one isn’t, but is gender neutral. English has no gender neutral third person, all we have is “its”. “Their” is incorrect, but is gender neutral. The second one is generally accepted nowadays, and is a better choice, unless your teacher is an older person–they probably grew up with the first one. Only the first is correct. SomeONE lost his, since “his” is singular, or one person. The best of both worlds is “Someone lost his or her blah blah blah”. It’s gender neutral and correct.
