Perhaps she should be concerned, but so should he. I don't know if they will investigate, but if they do both husband and wife will get into trouble. Why on earth did the husband report to the IND that he had knowingly entered into (what he calls) a sham marriage resulting in his 'wife' receiving ILR??
If you read the form SET(M) the part where both partners have to sign has very clear warnings about making a false declaration (bottom of page 17):
http://www.ind.homeoffice.gov.uk/6353/1 ... tm0606.pdf
I imagine the IND get a lot of 'tip-offs' and I'm sure in the past people on this forum have posted that the IND have not investigated as they get so many (a lot of which are just false/malicious), but a husband admitting that he himself was part of a deception - maybe they will investigate,..... they may also investigate whether the marriage failed after the ILR application and the husband is just trying to get revenge. Is your friend certain that he has reported this to the IND, maybe he is just being malicious?
In my opinion the husband has potentially put himself in a lot of trouble as well as his 'wife'.