Under U.S. law it could be obstruction of justice, contributing to the delinquency of a minor, and aiding and abetting a criminal.
The first two don't exist here. The third, I'd say it's wobbly at best. Even if we take the letter at total face value, that he was trying to stop the priest in question being defrocked despite knowing his criminal past, you'd have a hard time proving intent to aid and abet criminal action or evasion of justice. Being removed from the priesthood isn't a criminal sanction, so you can't 'aid and abet' someone to avoid that. You'd have to show that the then Cardinal intended to assist the priest in committing further crimes. A tough sell.
In fact, I'm racking my brains, and I can't think of a UK criminal sanction which applies to writing a letter suggesting a criminal not be fired. I may be missing something, but it's not coming to mind.
Maybe a 'causing or allowing' offence relating to child sexual assault, but again the burden of proof of what the Cardinal was aware of or intended would probably render that unlikely.
Certainly Dawkin's hyperbolic 'crimes against humanity' is ludicrous. Heck, if we locked people up for not firing subordinates who broke the law, most of our cabinet (actual and shadow) would be indictable.