"One man considers one day more sacred than another; another man considers every day alike.  Each one should be fully convinced in his own mind.  He who regards one day as special, does so to the Lord. He who eats meat, eats to the Lord, for he gives thanks to God; and he who abstains, does so to the Lord and gives thanks to God. For none of us lives to himself alone and none of us dies to himself alone.  If we live, we live to the Lord; and if we die, we die to the Lord. So, whether we live or die, we belong to the Lord." - Romans 14:5-8

When I read these words, I take it to mean that whether a person celebrates a certain holiday or not isn't important.  What is important is whether or not they truly belong to the Lord.  Your friend says they are a Christian.  Have they accepted the gospel?  Are they following the Lord?  These are the real questions to ask.  Someone can be just as godly a person whether they happen to celebrate specific days or not.

Perzik