It is not necessarily too late for her to say that her boyfriend stole the car. Should the matter reach court, the driver's girlfriend would have the burden of proving that she had no knowledge he was operating her vehicle and that the vehicle was in fact stolen. It is unlikely she can prove that she had no knowledge that he was operating the vehicle since she showed up at the scene with proof of insurance. Your wife should retain a local accident lawyer who can put pre-suit pressure on the insurer to provide coverage, and file suit if necessary. It is more or less an industry standard to provide free consultations, so go meet with an attorney and ask these and any other questions you may have.
You are not my client and I am not your lawyer. The answer given above is simply a general opinion and is not intended to be relied on for any purpose, it is not legal advice. The first and last advise I would always give on this site is to seek further advise from an attorney licensed to practice in your jurisdiction and who focuses on the area of law your question raises.
Your best option is to go to an attorney sooner than later.
You should know that this answer does not establish an attorney-client relationship. To do so would require a written agreement between us. You should be aware that there are time limits within which you must pursue any claim or suit.
I have seen this situation before, and the argument that the boyfriend "stole" the car does not usually hold water. The odds that he stole her car without her knowledge are slim, particularly if she did not report it to police. Even if he did steal it, if an attorney could establish that he used her car before, that could refute this argument. The insurance attached to the automobile that caused the accident would be primary and should cover this loss. I would retain an attorney right away to deal with the insurance company.
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