The concern is multifold. First, the tax rate you paid was less than you should have. Second and also, there are many rules on how you must treat income and deductions when you are married filing seperate which can substantially affect your tax return. While "fraud" may be a bit much as there needs to be an intent, you have filed incorrectly.
You will need to file amended tax returns for the years in question and there will almost assuredly be additional tax, penalties and interest due. Failing to amend the return before the IRS catches you could bring up the fraud issue, but I don't think someone in your situation would be prosecuted.
Actually, someone in their circumstances might not even get caught, but I just dealt with some clients over the last week in VERY similar circumstances.
It was the same thing...a divorce and the divorce brought to light the fact that they were not filing properly. The IRS got wind of it due to attorney requests for transcripts (with proper releases) to prove that the returns copies that were provided were accurate.
The IRS gave them 45 days to amend the original returns or they would consider the original returns to be fraudulent.
I had another set of clients recently that are going through bankruptcy, and again the IRS got wind of improper filing status through transcript requests...with similar results.
The IRS has hired about 15,000 new employees/auditors to specifically monitor these types of "cheating"....and has made their agents responsible for picking up on and reporting these kinds of issues (particularly if EIC is involved..which is almost always the case).
So fraud isn't necessarily a "bit much". Not any more. There are enormous quantities of people who have received EIC fraudulently and the IRS is working HARD to stamp that out.
When EIC is not involved its not a tremendous issue...but if they get wind of it they do "stomp" on it.