Curious how each department might handle this. Seems very specific to standards certain colleges or departments might take.
We had various speakers who were cancelled (both for graduate seminars and for a conference planned on NIH grant funds). Some speakers may have gotten a credit, others not. We viewed it as a sort of "lost cause" we'd pay for either way. Either they got a credit and would use it to come later, or they didn't get a credit and we still chose to reimburse them for unused airfare due to our previous commitment to do so. We encouraged reimbursing right away so that it was still charged in a timely fashion. I believe we did charge it to the NIH grant, whose budget included funds for a yearly conference. Seems NSF would be okay with this too (
https://www.whitehouse.gov/wp-content/uploads/2020/03/M-20-17.pdf).
As far as abiding by the 60-day reimbursement policy and how it applies to visitors (still not 100% clear to me in policy), I would certainly view this as a very strange extenuating circumstance. Our unit would likely choose to reimburse the visitor, especially if we had originally encouraged delay in reimbursement to "wait things out" for credit utilization.
Hope this is helpful, though I can see other units making different decisions.