Here's the issue.
You were paid to do a job as an employee or consultant.
Employees/consultants are NOT normally the recipients of these legal demands.
The focus is ALWAYS on either the company that actually owns the site, or the actual development company.
Masterfile has NO actual legal leverage to make you pay.
They'd have to sue the company or the web development company or BOTH.
If your friend's boss wanted to make you pay after MF makes him pay, he'd have to sue you.
This would be quite a difficult case, the costs would vastly eclipse anything that he'd gain, and he would be unlikely to prevail.
But, it's up to you... if you feel responsible, you can make the best deal that you can with MF and pay that.
However, if you don't have any money, what's the point?
I'd suggest Oscar's "letter program", BUT, I don't think that you should take steps that would imply that you are legally responsible.
I really think that this is going to end up in the hands of the company that owns the website.
S.G.