It's definitely not a bad financial or career move for either of them. I do think the optics are somewhat bad for OA. If he had just left to KY, people would be sad but would understand the move. However, coming here for 4 years to help resurrect a program, only to turn around and leave and also drag another integral assistant along with you is a low blow. This is a team you supposedly cared for during your time here, and the response is to leave and also stick another knife in their back on the way out? Of course, this may not at all be how things went down -- all of this is rumor at this point and nothing has actually happened -- but appearances can be just as meaningful as the actual events. Unless this is one big scheme to help your good buddy get paid (by Illinois) as you walk out the door, it looks and feels like he's lighting a match to the home he just helped finish building.
(FWIW, I'm of the opinion that we'll be fine -- while weathering a few bumps -- b/c BU is really good, but I do think how all of this is going down is really, really slimy. That's the name of the game though.)