72:13 (Formerly, people attended public bath houses; this halacha should be viewed in that context.) A man is not permitted to bathe with his father, his father-in-law, his stepfather or his sister’s husband. In places where it is customary to wear a towel when using the bath house, one may attend with these relatives. Similarly a student should not bathe together with his teacher unless the teacher requires the student’s assistance.
72:14 It is a mitzvah to wash one’s hair and to cut one’s nails on Friday. One should also cut his hair if necessary. There is a custom not to cut one’s fingernails and toenails on the same day. (Those who observe this custom should cut their toenails on Thursday and their fingernails on Friday – Mishnah Brurah 260:6.) A person should not cut his nails or his hair on Rosh Chodesh, even when it falls on a Friday. Some have the practice not to cut their nails in order; rather, they alternate fingers, starting with the index finger on the right hand and with the ring finger on the left hand. Some prefer not to cut their nails on Thursday because they will have visibly grown back somewhat by Saturday, which is less honor for Shabbos. Fingernail clippings should be disposed of properly (see Talmud Niddah 17a).