In D.C., a 12 year and 1 day old can consent with a 16 year and 0 day old. The D.C. law is just plain wrong and needs to be changed.
The best chance of a conviction is probably under Florida law, Criminal Law Section
847.0138, by the way, if any of the IMs were sent while he was in Florida. A federal conviction will be tough unless he went across state lines to actually have sex with a minor, did a video chat with a minor or has kiddie porn on his computer.
Irregardless of the legalities, what Foley did was sick, disgusting and immoral, and nobody here is or should be making excuses for him.