Safer to wait until you are back working, Since you don't know whether you will go back to this job, it is possible but risky to state your current income as the amount you are receiving in short term disability income. Your choice.
In my opinion, advising a joint sponsor for responsible and financially capable adults, is just not something I would do unless it is really necessary and the person has easy access to somebody willing to sign that contract and take on the associated obligation. Many of us, are proud of and attached to our adult independence. It's much easier to type the words "joint sponsor" than obtaining and using one is.