"Someone is ahead of you."
Translation:Jemand ist dir voraus.
That would be 'Someone is in front of you' (i.e. being ahead in a more concrete, spatial sense). In some contexts, if 'ahead' effectively means 'in front of' (I'm talking car races here), you could use 'jemand ist vor dir' for 'someone is ahead of you'. So, if speaking figuratively 'voraus' is the better choice.
The following is according to a native, but they tend to don't have a clue how German works. In any case: voraus is relative, vorn(e) is absolute. Hence in a run, a runner is ahead of all other runners (relative), and in front of the run: er ist im Rennen vorn - er ist allen voraus. HTH!
The preposition vor always requires dative case. The dative equivalent of Sie is Ihnen, so that sentence should be jemand ist vor Ihnen. However, as explained in the other comments, the better sentence structure is jemand ist dir/euch/Ihnen voraus because it means "ahead of" and not just "in front of", which is what your sentence means. Another competitor could be "ahead of me" on a racecourse, but because of the way the track curves around, they are actually behind me to my left, for example. In that situation your sentence would not be accurate.