Mr. Difficult was thisclose to being formally written up and on his way out the door for insubordination, lack of integrity, and absences, when he got real, honest to god, bad Dr. news and may need more time off. What can I do or not do to keep him on the getting-replaced track without tying the medical thing up in it? I am not a jerk 
so I feel terrible about Mr D’s problem 
but I have work to do and when he is here, it isn’t getting done.:mad:
This has the potential to be horrible news from his doc, but we don’t know yet. Could be a hangnail, could maybe kill him, we have yet to learn. Could be out a week, could be months, I don’t know. For all I know he is at Disneyland and I have been punked.
I assume, and hope, you have a formal written record of the job discussions with him, previous yearly job evaluations (with poor ratings and clear-cut improvement goals that he did indeed fail to meet), and so on?
the previous manager an average review for last year with no specific comments. gee, thanks, previous manager, I owe you one.
I don’t know if HR has anything specific enough to nail him, but several of my colleagues have mentioned incidents or issues which I have been documenting the past few months since I took over. Nobody respects him, nobody hides it. we have been working to get enough documented but I am not sure we are there yet.
Is he bulletproof now that he has a doctors note?
If you’re the manager, go talk with HR and stop asking strangers on a message board. It’ll depend on local/federal laws, company policies etc etc etc. They’re paid to know.
Potentially, for a period of time. In the USA there is something known as the Family Medical Leave Act. It is intended to protect a persons employment while they are out. This is so managers can’t just fire you because you are sick (and not able to work). However, there are limitations. Talk to your HR folks, they should know all of the minute details.
Editorially speaking, Mr. Difficult is potentially aware of his protection under FMLA and actively sought a doctors note - aware that he was near to being fired. It is a common trick. His next play would be to go on some sort of long term disability (much harder to accomplish without a willing accomplice doctor).
All that being said, you and HR must be perfect and pure in your dealings with this, lest Mr. Difficult has any ammo for coming back at you and cashing in. Your posting here on this SDMB would be subject to discovery and evidence in a court case - so keep that in mind. Even you referring to him as Mr. Difficult would be trouble for you. 
I know. At least I have them to advise. I just don’t have their answer yet and it’s been a bad week.
Damn the not anonymous interwebs! I was not here.
Seriously, thanks for the heads up.
Assuming he’s full-time and has been with your company for at least a year, he’ll be eligible for FMLA. And hopefully, someone at your company has notified him of his FMLA eligibility in writing - so whoever administers FMLA for your company needs to know about his medical absence ASAP.
Even if his FMLA leave is approved (he’ll have to provide medical certifcation to the leave administrator), he’s not bulletproof if you can prove that his termination is completely unrelated to his FMLA. But if your documentation is not completely solid, you’re probably much better off waiting out the FMLA for 12 weeks (you don’t have to pay him). You can’t fill his position but you can hire a temp or something if you need to. If and when he comes back you can pick up the disciplinary process where you left off as if the leave never happened.