Option 1

Talk to Alex privately, express your concerns calmly, and explain how their actions are affecting the party planning and potentially the guest of honor's experience.

Option 2

Involve another mutual friend to mediate or confront Alex on your behalf, or threaten to tell the guest of honor about the surprise if Alex doesn't cooperate immediately.

Option 3

Do nothing and hope Alex realizes their mistakes, or just take over all the planning tasks yourself to ensure the party happens, even if it means doing everything alone.