If a friend uses my home address for his tax return, and he owes the
IRS tons of money, will I be liable in any way?
Assuming that this is an income tax or sale tax return, rather than a property tax return, generally not.
The only case where you could be liable is where you were conspiring to help him evade tax collection somehow, or conspiring to evade state sales or income taxes.
For example, suppose that your friend had no connection to your home in your state (Nevada) with no income taxes and has never even set foot in your state, and actually lives in a state with high income taxes (California). But your friend is using your address to falsely claim residency in Nevada with your knowledge and cooperation to avoid paying California income taxes. To make it spicy, let's assume that your friend's state income tax liability would be $400,000 in California, but $0 in Nevada. In that case, you might have felony criminal liability under California's tax evasion statutes (for which you could be extradited to California) for conspiring with your friend to evade his California income tax liability.
But if you had no idea of what your friend was up to, and simply agreed to forward mail to him at your address (the way a typical mail drop company does), you would typically have no criminal or civil liability (subject to the "willful blindness" exception to lack of knowledge where you strongly suspect that your friend is up to no good but take a don't ask, don't tell approach).
Also normally would it be a problem if he filed a tax return with my
address, and I filed a tax return with the same address in the same