While
Frenetic Efreet's printed text may talk about the opponent calling the flip, the printed text doesn't actually matter, because the wording that matters is the official Oracle wording as found in its Gatherer entry, which does no such thing–it reads:
{0}: Flip a coin. If you win the flip, Frenetic Efreet phases out. If you lose the flip, sacrifice Frenetic Efreet.
The player who controls the ability (Alvin, in this case), as the affected player, is the one who calls it, and they're the one who wins or loses it–no other player is involved.