It's an ability though, and calculated in a separate stage than, say, the boost from a swords dance is. I seriously doubt that foul play would use both the user's and the target's abilities.
I know it's from 2 weeks ago, but because it hasn't had been properly explained, I will explain.
Foul Play is a Dark-type move that uses the target
natural Attack stat, including its stat changes. Multipliera that affect this move that aren't actually stat changes are taken differently.
In other words:
-1) it uses the actual Attack stat of the Pokemon. The user natural Attack stat has no impact.
-2) If the Foul Pay target uses Attack boosting moves (examples; belly Drum, Dragon Dance, Fell Stinger, Growl, Hone Claws, Howl, Meditate, Power-up Punch, metoer Mash, Shell Smash (be warned) Shift Gear, Sowrds Dance and Work up boosts in the opponent raise Foul Play's power).
-3) If the Foul Play target is affected by Attack lowering moves, the power of foul Play is being reduced. Charm, Feather Dance, Growl (yes, and has a niche in doubles), Memento Parting Shot, Play Rough and Tickle.
-4) If the Foul Play target is affected by Swagger, the power of Foul Play is raisen.
-5) About Abilities:
-Adaptability: User (no legal Pokemon has Adaptability + STAB Foul Play).
-Analytic: User
-Contrary: Opponent
-Dark Aura: User (Yveltal learns this move,
-Defeatist: User
-Defiant: User
-Flower gift: User and allies
-Guts: User
Huge Power/Pure Power: User
-Intimidate: Opponent. (the ones who receives an Attack drop).
Moody: Opponent
-Moxie: Opponent
-Parental Bond: User
Rivalry: User
-Sap Sipper: Opponent
-Simple: Opponent.
-Tinted Lens: User.
-Toxic Boost: User
-6) Rest of factor:
Items:User
-Dread Plate/BlackGlasses: User
-Choice Band: User
-Life Orb: User
-Dark Gem: User
-Muscle Band: User
-Liechi Berry: Opponent.
Other:
-STAB Bonus: User
-Burn status: User