Congratulating someone else on the occasion of his or her birthday is a kind of participating and cooperating in this innovation, and imitating the disbelievers.

There is no sahih hadith concerning the virtue of the night of the fifteenth of Sha’ban. With regard to Allah’s descending on the 15th night of Sha’ban, there are hadith which are weak, but the reports that He descends every night are proven and sahih, so the night of the fifteenth of Sha’ban is included in that. For more, please see the detailed answer.