Yes, if you are talking generally about someone and you don’t know their sex.

The answer to your question used to be NO. But today, more and more, the answer is YES. Traditionally, the male possessive pronoun was used in situations where a gender was not specified. However, the feminists objected to this in the U.S. and made a big effort to change the custom. In some languages, like Italian, male forms are still used when gender is unknown. For example, in Italian if you say "John has three brothers", those "brothers" could be boys or girls. In English this is not the case. A brother can only be male. To my own ear it is grating to hear "their" used in this way because it is grammatically so absurd - "their" is plural - but that's the way things are nowadays.