As a Muslim, I know since I was a little kid the prestigious positioning the Christ (Al Masee7 Ebn Mariam) has in Quran and Islam, and how he is among the most prominent messengers of Allah (2olo el 3azm). I also know that Virgin Mary (Mariam Al 3azraa) is one of the most sacred ladies in Islam. Moreover, the birth of the Chirst was positioned as a divine miracle more than once in Quran. So Islam does acknlowge the birth of the Christ and the miracle behind it, so what is wrong when people congratulate their Christian friends about it?!
Even if we have different points of views about the nature of Christ, this doesn't deny the fact that it is his birthday afterall, and on his birthday -peace be upon him- I am telling all my Christian Orthodox Readers today... MERRY CHRISTMAS :)