USDA mistake or not, she knew she should be making payments and just didn’t. Even if you take this to court, no judge will buy that she earnestly believed she didn’t have to pay. USDA isn’t going to work with her; it’ll likely require a huge sum (8 years of back payment plus fines and late fees) to stave off foreclosure.
Honestly, your best bet is to try to sell the house as‑is within the next 45 days, before foreclosure, and hope she can walk away without wrecking her credit tough if her ex is still on the mortgage and won’t agree. If you do sell, don’t get greedy; just aim to pay off the mortgage and spare her credit for the next ten years.
Don’t combine finances with her her presence on any lease or loan will be toxic.