How does a bank become a surety?

for example P local tycoon was keen to invest in Ultra. ( a well known company.) He enquired from Ultra's bankers, G Bank Ltd, about Ultra's credit-worthiness and credibility of their investment programme. Stanley Hewitt, the Branch Manager, wrote back to say Ultra had an impeccable record, and that Ultra's products were genuine and reliable. P then asked that the Bank becomes surety and co-principal debtor in respect of E500, 000 which he intended to deposit in Ultra for three years.