Since MEF, I'm not sure how long it takes returns to post to the Master File in Martinsburg, which then tells Treasury to issue refund checks.  But if the original return was filed just last week, I would file the superseding return now, hoping that it would freeze the account.  If it doesn't, I would tell the client not to cash the check, but to return it to IRS -- unless they don't want the hassle and are willing to pay interest on the amount paid back, when the refund check arrives.