My opponent has Mask of Foresight in play. They move a unit to a battlefield where I have a unit. Before a showdown occurs, they cast Ride the Wind to move to another battlefield. Does their unit get the Mask of Foresight bonus for both battlefields or are they not considered an attacker at the first battlefield?

Yes — the unit gets Mask's +1 at the first battlefield and can get a second +1 at the second, +2 total (assuming the second destination is again a battlefield where a new combat opens and their unit is the lone attacker there).

The key point is there is no "before the showdown" window to Ride the Wind in. The Showdown opens automatically in the cleanup at the end of the move, and the Attacker designation is locked in the moment combat opens — the moved unit becomes an Attacker, Mask's trigger fires and goes onto the combat chain immediately, before anyone gets priority to cast anything (CR 466.2.c.3, 466.2.d). Ride the Wind is Action, not Reaction, so they couldn't respond to that trigger anyway; it resolves and the +1 sticks to the unit for the turn regardless of where it later goes.

Then when Ride the Wind resolves and moves the unit, it drops its Attacker designation at battlefield 1 (CR 323.2.c), which ends that combat — but arriving at the new battlefield applies Contested and stages a fresh combat there. When that combat opens, it gains Attacker there, and Mask triggers again: once per unit per combat, so a new combat is a new trigger. Only way they dodge the second one is if the second battlefield is one they already control (no Contested, no showdown) or they're not alone when that combat opens.