As the article notes, US and Mex have bilateral extradition so it’s doubtful she intended to stay there to escape prison. She was likely going on to another country after that.
>Holmes’ partner, William Evans, also bought a one-way ticket “and did not return until approximately six weeks later, returning from a different continent,” prosecutors said.
Although if I had high resources and was fleeing justice, I probably wouldn’t buy a ticket ahead of time. If anything I would book some domestic travel to be on flight manifests.
There are plenty of countries that won't extradite their nationals as a matter of policy like China, Russia, Switzerland, Lebanon, and so on (it's most of the world, probably), but the vast majority won't bat an eye to throwing her back to the US based LEO wolves.
It's thought for instance intelligence may know wanted British terrorist Samantha Lewthwaite is in Kenya or Somalia, but impractical to extract due to being controlled by Al-Shebab. Nobody is gonna send the troops/3 letter goons into some no-mans contested land over some non-violent holmes like character.
Who in their right mind enters a flight manifest, let alone an international one, as a high profile wanted person.
A decoy could take the trip to Mexico for you...
Rich != Smart