You may absolutely keep good relationships with non-Muslim friends and treat them with kindness, justice, and good character, for Allah does not forbid you from dealing kindly with those who do not fight you over your religion. What you avoid is participating in the actual religious rituals of another faith or affirming beliefs that contradict tawheed, since the Prophet warned that whoever imitates a people is counted among them. Sharing a meal, giving a gift, or being present at a social gathering is different from taking part in worship or celebrating the religious act itself. So keep the friendship and the good manners, but keep your worship for Allah alone.
Evidence — the daleel
On deen2u
Informational, not a personal fatwa. Consult a qualified scholar for rulings on your situation.