I believe this assertion is not entirely correct. Here is the wording directly from Govt. of Canada's website:
You may need to file taxes in Canada for at least 3 years during the 5 years right before the date you apply.
That's correct. If a person made no income or income less than the reporting threshold, then they would not have to file tax returns.I do not believe that your friend will need to wait to file for citizenship application. Filing of taxation, if required, is the requirement. Typically they'd like to see you file for each of the 3 years (out of 5), but if you did not have to file a tax return for 3 out of the 5 years, there is no reason that you cannot apply for citizenship with 2 years of tax returns.
I assumed a large majority of citizenship grant applicants posting here either work and earn at least minimum wage that'll cross the reporting threshold OR claim benefits (HST return, CCB etc) that would require them to file tax return ; hence that statement.