Because the event itself is after the end of the grant, I would disallow charging it to the grant. Even if certain expenses are
paid for before the grant ends, the actual conference dates are after grant end dates so those charges would not be allowed. If it's possible a No Cost Extension can be pursued, that would be optimal.
I've only deviated from this in one case with sponsor approval, that I recall. It may have been a publication charge or website cost that was pertinent to the grant accomplishments (and for some reason couldn't be charged within the grant dates).