You should win a "good samaritan" award for helping that old lady out like you did. I don't think you have to take all the blame on yourself. Just mention to her that you think her muffler may have been rusted out, because something broke off when getting the push out of the snow. Tell her she probably needs to get a new muffler. After all, it sounds like you don't charge her for the work you do so she certainly should understand.