What happens when a sham marriage is discovered?

Some years ago my now fiancee entered into a marriage of convenience with a friend in order to allow him to remain in England. She is a European rather than British citizen and left the UK shortly after.

Obviously I would like to marry the girl but the guy is understandably reluctant to divorce. Not to put too fine a point on it, and although I have some sympathy for him, I find this to be an unacceptable inconvenience.

Given that my fiancee has been living and working in a different county for almost their entire marriage it would be easy enough to show this to be a sham marriage. I am curious to know what the consequences would be were this discovered.

I would guess though that this information would lead to legal difficulty for my fiancee and / or would not guarantee a speedy divorce. Am I correct? Has anyone any advice?