Is it permissible for a wife who is not nidah to cut her husband’s hair in the privacy of their own home?
When she is not a nidah it is permitted.
However when she is a nidah she should not give her husband a hair cut, because she is going to touch his hair, and even if she will be careful not to even touch his hair she is still touching him through another object, which according to most poskim is forbidden.
Taz Y:D 195-8,Beer Moshe 1-50 (18) and 4-113-7, Knei Bosem 1-86-2, Pischei Halacha (Artscroll) vol. 2 pg 37.