This winds me up! I think taking a place that should go to a child who genuinely needs it, is both immoral and totally dishonest. Just because the system allows parents to do this, doesn’t make it right. It’s would make me feel very uncomfortable and I question your friend, or her husbands ethics.
Having said that, I am not sure how you should proceed. If you were that sure about saying something, I wouldn’t say anything unless anonymously, but you would also have to consider that IF she hasn’t told anyone else except you, then she will know who has told the school.
I guess it depends on how much you value your friendship, and whether you will be able to bite your tongue if they ever mention it, or about why they have supposedly been offered a bursary.
Personally we sent our kids to private school and made lots of personal sacrifices to do so, and would never have considered trying to scam the system. I don’t see your comments as being jealous, rather than indignant!