Is it sinful to eat food bought by a girl who practices prostitution, and what must be done to atone for this sin?

Celebrating Christmas is a forbidden innovation (bid'ah muharramah), and participating in it is prohibited. One must repent to Allah for attending such celebrations. As for eating and drinking from the colleague's money, it is permissible with some dislike (karahah) if its lawful part is mixed with its unlawful part. It is preferable to avoid it as a precautionary measure.