How to get refund of last 4-5 years in case of death of assessee

First of all tell ur friend to go and meet his mother's ward AO. To get refund you have to file income tax return. You can file IT return from AY 2009-10 to AY 2013-14 under 119

-condonation of delay. Tell AO that yur friend's mother didnt know TDS was being cut and the taxable income is less than prescribed limit. 

The legal heir can file return on her behalf and claim refund.

let ur friend appoint a good CA . He will do all the letter writing and meeting the AO and filing of return. AY 2014-15 and 15-16 return can still be filed.

Nothing to worry. Take a xerox  copy of both accounts and show that it was an internal transaction. File IT return for AY 2012-13 and 13-14 in response to Notice and claim TDS to get refund. 

While Signing IT return the legal heir should sign. Your friend  may be required to give sworn affidavit to prove he is the legal heir of the assesse.

Take help of good CA. He will help you in writing letters or visiting AO and explaining.

If the total fees of CA is far more than refunds , tell ur friend to do it himself.

cash deposit is not a matter, if you have enough evidence to show that such cash was withdrawn and deposited from one a/c to another,

just meet a ca nearby and they will make letter and submit with copies of bank transactions copy