The creditor can sue her and get a judgment. With a judgment, which would have to "domesticated" in MA if the judgment if entered by a FL court, the creditor could try to levy her bank account. Make certain that her bank knows that her only income is from pension income plus SS. Generally speaking, pension and SS income is exempt from a bank levy as long as the amount in her bank account is not over two months worth of such monthly income.There is probably some kind of form that she may need to fill out and file with her bank.
Retirement accounts generally are exempt from garnishment/bank levy as well.
Whether the creditor will actually go after her is anyone's guess.
You idicated that your MIL's cancer is terminal. The creditor can file a claim against her estate when the time comes. The executor or personal representative would have to deal with it. However, any IRA, if not depleted at death, would pass directly to the named beneficiary without going thru probate. Your wife (I assume that she is probably handling these matters), also may be able to set up her MIL;s bank account so that it is payable to your wife once her mother dies. That would eliminate the need to any probate proceeding for that asset as well. I don't know if your MIL has any other assets that might have to go thru probate. That is something that your wife probably should disucss with her mother.
I would be tempted to ignore these letters. By the time, the creditor actually goes to court, gets that judgment, etc., the chances are good that it is her estate, if she has one, that will have to deal with the issue. If she does not have any other assets and there is no need for probate, the creditor will just have to "eat this one". Your wife is not personally liable for your mother's debts unless she cosigned that loan with her or she is the exetress/personal representative of her estate when the time comes and she chooses to not deal with the matter at that point.