How does a bank investigate fraudulent activity with a bank account

[hopelesslycynical]

A close relative has for some time been a victim of fraudulent activity, money going missing, expensive and random items being bought e.g a wedding dress in Russia etc. Each time she has notified the bank and been reimbursed. She even changed banks and it continued with the new account. The latest incident has occurred and the bank have said her partner is responsible. He has denied it, so she is unsure of what to do, who to believe. She is vulnerable with a long history of PTSD and other mental health issues, and of course this is having an effect on my parents. How thorough are bank investigations and how do they know the perpetrator is him?

I must say that as she has even changed banks and the transactions still continue then it seems highly likely to be someone she knows.

Are the transactions made using her card? In which case the perpetrator will know her PIN and the 3 digit card security code on the back of the card.

The bank will look at all the transactions, how they were made, when they were made etc and make a decision based on that. As I said above, I can easily see why they have come to the conclusion that it is her partner.

With a bank change it is obviously her partner - he doesn't need the pin to buy online, just the card and address details.

The bank will stop reimbursing. If she won't throw him out, then change the card.and make sure he doesn't see it.