Option 1

Speak immediately with your boss to explain the conflict and collaboratively find a solution, perhaps by attending part of the party or ensuring your presentation responsibilities are covered.

Option 2

Inform your direct manager (who is not the retiring boss) that you will be skipping the retirement party to prioritize the client presentation, assuming they will handle any fallout.

Option 3

Attempt to briefly appear at both events, trying to split your time between the client presentation and the retirement party, hoping neither party notices your divided attention.