I have a client who came to my office to amend form 1065. After checking and doing my "Due diligence" I found out that his partnership taxes for 2018 and 2019 were filed wrong as he ended his partnership on 2017. Now here my question.

If he already filed his tax return and made the mistake of filing it as 2 member LLC. when I know he should have filed as a sole proprietorship. How can I fix this. Just to mention that it was filed wrong for two years 2018 and 2019.

Thanks