Forgive all these people who have Answered your question, and were not of help, although, there was a few people who were. When you were done with this spirit, you should have said goodbye, and it should have left, being as this was not done, it will be more difficult to remove. also, your friend should have kept the water nearby, but not quite in her hand.
The first thing you should do now, is have your friend, and yourself get back on the board, and ask it to leave, if it refuses, remind him that his life is over, and that his place, is where he was. Remind him, that it was the way he lived his life, that put him in the circumstance, he is in now, If it is needed, recite this quote....
"By that what has been placed in me by the almighty, I command you in the name of Jesus Christ, to return to your Place of which you have come. begone in the name of Jesus Christ." This should free you both from it's grip.