Nah. It's all about knowing your limits. There's only so much one man in his garage can do.
I'd consider offering them usability advice/consultancy, but get the impression they don't take it seriously enough to care.
As for coding a whole online banking system. The idea fills me with fear. Hey, I'm good, but I know my limits. I wouldn't want the burden of any bugs. I like the kind of bugs of which the consequences are explained away and then easily fixed.