Legal - Yes. If caught out fines are up to $300k and possible jail time. The person seeking the visa would be booted and banned for as little as 3 years and possibly a lifetime ban from travelling to the Country, and any government payments received would need to repaid as well, which if she's been kicked out you can guarantee she won't be stumping up her half.
Financial - if not caught then your newly betrothed may well seek her half when you divorce. Of course the joke would be on them if you only own rubbish and you'd gladly give half of it up for 2 years of free sex. Obviously some sort of prenuptial agreement would need to be agreed to.
If you are caught, yeah fines and repayment of any tax breaks, payments etc received from the fake relationship status.
Social - Well that's up to you and your social circle.

The above statements in no way constitute legal advice and I strongly encourage you to undertake your own enquiries on the matter. Sure some people get away with it, but the govt has been cracking down big time and if they get a tip someone is pulling the fake marriage for visa scam they tend to take it seriously. A mate of mine married his legit Thai gf, they'd been together years, and dept of immigration made me sign a series of stat decs verifying their relationship details, where they met, how long they'd been together etc.