A new client came to me, wanting me to prepare their 2018 tax return. They gave me everything, including their partnership K-1 and a W2. I didn't look too deep into at the moment, but now that I'm working on their taxes I realized that the W2 was for their partnership, which I know is a no no. After discussing this with my client I've learned they've been doing this the past 3 years and have prepared the taxes themselves.
My questions is how should I address this? I've talked to other tax preparers and got mixed answers. Some say amend everything for the past 3 years but explain the costs and mess it can create (which I have). Then some say just file 2018 as is, but explain going forward this can't be done.
I'm conflicted on which route to take.