The Gemora allows special leniencies for a Bogeres from the restrictions of mourning. This law is brought in Shulchan Aruch (Y.D., end of Siman 381). Why is it omitted by the Rif, Rosh and Rambam?
Good question. We do not yet have an answer. The Sefas Emes asks your question but he does not give an answer.
Moreover, the RAMBAM does not cite at all the prohibition of Kichul and Pirkus (which is where the leniences of a Bogeres apply according to the Gemara's conclusion). The RAMBAN writes that the source for the prohibition of Kichul and Pirkus is the Beraisa in Moed Katan (20b), and when the Rambam records the Beraisa, he changes the text from the way it appears in the Gemara, while the Rif and the Rosh in Moed Katan record the Beraisa as it appears in the Gemara, but they leave out the Halachah of Bogeres. (The Rambam, in ch. 11, cites the Gemara in Kesuvos 4b that says that we do not prevent a Kalah from adorning herself during mourning, which implies that everyone else is prohibited.) This question requires further elucidation.
Yosef Ben-Arza