Foul Play is a move that works differently than other normal dark-type moves. In foul play, it's essentially like if the target is attacking themselves - since their power is used against them.
In attack, metagross beats sableye by a lot. Therefore, sableye's foul play would be much stronger than metagross's. Using intimidate lowers your opponents attack. This will affect your foul play's results. Using intimidate is essentially lowering your foul play's attack power.
Even though metagross has much higher attack, sableye's will be stronger. So in your case, scenario 1 would be more powerful. If someone used intimidate on metagross before foul play (obviously intimidate would activate first), then your foul play's damage lowers by one stage. If your opponent uses any attack increasing moves like "Swords Dance", then it will increase the power of foul play. Anything that happens to sableye wouldn't count - all depends on your opponent.
Hope I helped! :)