Celebrating birthdays

Q: If someone follows the opinion that celebrating birthdays is halaal as long as they are not extravagant and as long as they don't include something haraam, then if that person says happy birthday to non-Muslims who are celebrating it, are they still Muslim?

Because I've heard from people that if a Muslim participates in a kaafirs events then they have committed kufr (atleats that is what I understood) but what if that person thinks its ok to celebrate birthdays?