I have a friend who is on probation for minor pot/DWI offenses and is considering moving to Spain or Mexico?

He is of Spanish and Mexican decent but born in the US, the offenses are Class B misdemeanors, he figures financially it would just be easier than to deal with probation, the money involved, and the legal system here in the US. I know that there is no conviction on his record as of yet. I think at this point he's spent over $20K in fines, bail, probation fees, lawyers, etc. We all know that probation is only about money. And I know that he can qualify for a ancestry visa in Spain and Mexico. If he renounced his US citizenship, could / would the US pursue him on a Class B misdemeanor? I don't see how the US could pursue someone that is not a US citizen anymore. But then again I don't know how international law works either. What's the community's take on this? I need real answers and not a berating on "How bad it is to run", or "You did crime now do the time." BS, real answers only, I happen to agree with him on moving, he has paid his debt of $20K plus and they just want more, and he's done jail time because on occasion he couldn't pay his P.O.