I was given some horrid advice from a professional tax adviser and want to know if anyone has had any experience with legal action. Can anyone help?

Any caveat in the small print of the contract you signed? If you signed one?

normally excluded in the T&C of a tax (or any) advisor. if he did something without your knowing you may hold him responsible but the T&Cs would in general limit your claim to the fee you paid him. We would need more info on the real terms and contract and what went wrong.

Maybe the tax office will let you change your statement?

Swiss taxes, or US taxes?