Answer: I would listen to the strategies that both sides have, and then reach a compromise that they are happy with after adding some of my own suggestions.
Explanation: Picking a side will make the other person whose strategy wasn't chosen feel angry, which intensifies as you remember that these two do not get along in a professional setting. Therefore, saying your thoughts and what you think will be a good strategy for a graduation celebration first and listening to what they have to say, and then possibly incorporating parts of their own strategies so that both sides have original ideas that are included in the final strategy.
