What should the boss say at an employee's wedding?

If you are all very friendly and on a first-name basis , and all the other employees feel the same and like and respect her as a fair and good boss, sure, why not?

If you think she is Cruella de Villes younger sister, then no, that would be hypocritical. Do RSVP in any case so she can plan accordingly . A lovely card is all tht is required. A small , tasteful, but inexpensive gift if you feel like it, but not mandatory. If she is rude enough to ask why you don't want to attend , there is no need to be specific . Just say you were otherwise engaged . She has no right to pry personal info out of you.