Bid around the days you need off. If that doesn't work, see if another FO will trade days with you. Attempt to drop your work days, provided there's enough coverage. At my company, we can do a bad day/worse day type thing - call Crew Scheduling and offer to trade days for other days they may need you more than the ones you're scheduled for. If none of that works, explain your situation to your Chief Pilot - sometimes they can approve a personal drop... If all else fails - buy a really expensive gift and hope your buddy is the forgiving sort.