Is this way of claiming deduction correct?

I know a lady whose income is from small chores and tailoring.She earns somewhere around 2 lakh 20000 to 2 lakh 50000 per annum.

She doesnt contribute any money from this income to any financial instrument as her financial knowledge is poor.

So a tax liablity is due of 3000 to 5000 on her.

She doesnt  file any  return thats another story.

But her husband have open a ppf account on her name and contribute 50000 to that annual via cash.Also the same he does for same.

They dont file return .

I was thinking the lady can claim deduction over the tax  due.

But wont that be an invalid way  to claim deduction?

Because that 50000 is over the 2.20l amount whic she earns.But  when filing tax return only stuff seen is whether any contribution made to 80c instrument and if yes  thats 50k.so from income of 2.20l that gets  reduce.And return goes to nil.

Or is it that while filing return one also have to mention whether one paid via cash or cheque to ppf in return?