There is just no need for it. Practically all the people I know have had some kind of birthday in lockdown this year. It is really that difficult to stay away from people? Would Harry have been scared for life just by spending the day with his Mum, Dad and brother?
Personally I couldn't live with myself if I unwittingly passed on this virus because I felt the need to have people in my house for a seven year olds birthday and her sister runs an events business. Does she not realise that the longer people don't listen to the rules the longer lockdowns and restrictions will be in place.