thanks yogesh and kvijay.
Yogesh your wife is lucky for getting such grandma. she could convince. At this age after carrying all traditions for entire life for mom it’s not possible to convince someone.
as vijay said dad is key. and she could convince him at least to allow her to wear nightie. vijay you are right. she should start with simple cotton gown/maxi. She may buy it today or tomorrow. after getting permission she is not showing eagerness to wear it to keep dad happy. She will buy it in one or two days. If any of our friends having any valuable suggestions please suggest.