Money Lending

Hi. My father who was a tax practitioner had a client for a long time, and I am not aware of any professional dealings between them. My father has since expired and this client now tells us that he lent some money with some % interest rate to my dad. When we asked for any proof/evidence, he went to court and filed a case against my mother, my 2 brothers, and myself. We have hired a lawyer and he is due to represent us in court to understand the basis of this case but I am wondering what documents are treated as evidence of money lending according to IPC. Since he was a client, he and his family have had numerous checks written to my dad and he, in turn, has also written some checks back to them for reasons unknown to us. I am guessing the client is using this as "Evidence" stating the money was lent to him but how do we believe this is true -- and how much of this is true since our dad never discussed this lending business with us before. We would greatly appreciate any suggestions/thoughts on the matter. thanks.