Frag grenades require you to select a battle mat position where it will be thrown. The grenade will deal its rolled damage to that position, and half damage to orthogonally adjacent positions. The position she throws it to doesn't even have to be occupied. 

However, "target" is a very specific word in this game. Frag is not a targeting skill, and a battle mat position cannot be a target. Being ranged, Boomer can select any Baddie on the battle mat as her target, and this will be the Baddie she applies her Atk damage to as well as any Backup Plan that uses the word "target." Boomer's grenades and other skills do not use the word target, and can therefore affect Baddies that aren't her target. She can, for example, choose one Baddie as her target for Atk Dice, throw her Frag to a different position, and the Stunner grenade on yet a different Baddie.