First of all, best wishes to the candidate. Legally, this is a really tricky situation.
The bad news is that you can’t simply revoke the job offer because of an injury since that would likely be deemed disability discrimination. Instead, you need to accommodate the candidate to the point of undue hardship. Whether waiting 2 months would impose undue hardship depends on the situation–what’s the position, can it be left vacant for a while or does it need to be refilled, what’s the candidate’s realistic prognosis–is it 2 months or more like one year, will the candidate even be capable of doing the job after the injury, etc.
The good news is that because the candidate isn’t an employee and hasn’t put in the minimum time of service, his or her employment standards rights haven’t vested. Translation: No termination notice, accrued vacations or any of that.
Theoretically, you could also have obligations under the terms of any contract or offer letter that the candidate signed on the remote possibility that the contract or offer addresses or applies to the candidate’s pre-employment rights.
Hope that helps and get well soon, Mr or Ms Candidate. Glenn