what do you say to someone who celebrates eid

You can keep it simple and warm. The most common thing to say is:

• “Eid Mubarak!” – which means “Blessed Eid” or “Have a blessed Eid.”

If you want to add a bit more:

• “Eid Mubarak to you and your family.”
• “Wishing you a happy and blessed Eid.”
• “Hope you have a peaceful and joyful Eid.”

If it’s a colleague or someone you don’t know very well, something like:

• “Eid Mubarak! Wishing you a wonderful celebration.”

All of these are respectful, appropriate, and fine to use even if you’re not Muslim yourself.