1.Ask, in a way that makes "nothing" an easy answer
Not "what do you want to do for your birthday", which reads as a request to produce enthusiasm. Try "I would like to mark your birthday, would you rather have dinner with two people, one quiet thing with me, or genuinely nothing." Three options with an exit is a question somebody exhausted can answer honestly, and a person who has been saying yes to things they cannot face will take the exit gratefully.



