My friend gave me her Canon 28-70 f/2.8L lens a few days ago since it couldn't be repaired by Canon Factory Service.
She had dropped her 5D with the lens on about a handful of times over the years.
3 of the 4 screw tabs (the tabs that the silver lens mount is screwed to) were broken and causing the lens to separate right behind the zoom ring.
After figuring out how to take the back of the lens apart, I glued the tabs back on and they seem to be holding fine.
there is still an issue with the telescoping section, it has a little side to side play in it and I have no idea how to really get the lens completely apart to see how to fix that.
Other than that, the lens still works great.
here it is on my 40D,



