Unfortunately, it's not possible to reverse an action for a request, but accepting the return isn't necessarily a bad thing to do (by which I mean, although it progresses the request, it also means any further action has to be taken by the buyer, whereas by not doing it, the request officially awaits your action).

Requests can only be closed by the buyer (or by eBay if it's escalated), so I'd send them a message outlining that you actioned the return, however returning the item isn't required and ask them to to close the case once they receive the replacement.