I can only guess what Seagal could possibly teach an elite boxer, but with his Hapkido I would assume it would involve anticipating punches or small movements to avoid them. I dunno lol.
Seagal is trained in Aikido, not Hapkido. If it was Hapkido, he may have a new strike or something, but all Aikido could really offer a boxer would be punch avoidance, something I'm fairly sure a guy who makes his living getting punched in the face would work on anyways.
“A black belt only covers two inches of your ass. You have to cover the rest.” - Royce Gracie