My client has been a US citizen for many decades. He has property in an European country. The European country wants proof he filed and paid taxes in the USA for each and every year (many decades). What should we ask for from the IRS? Thank you.

I guess NOT much; There are many reasons why you should keep copies of your federal/state tax returns. For example, you may need them to answer the inquiry form the foreign taxation jurisdiction. I do not think that the IRS directly submits any proof he filed and paid taxes in the USA for each and every year (many decades). ALSO, I don't think the IRS will be of much assistance in providing you with past returns. It may be 3-7 years that they can look at your returns for errors and audits. I don't know for sure, but you may want to contact an IRS office about what they are willing to dig up for you.