Just remember.

The choice to work with "difficult clients" is yours.

Most Accountants Liability classes that I have taken, stress that "difficult clients" also tend to be the highest liability risk.

Tell them to take a hike. I the time you spend researching/worrying about this, you could have completed a few other returns.

Also - It is not your responsibility to find another preparer for work they ask for that you do not normally do.