Does this help?
IV) BINDING CONTRACT EXCEPTION.—In the case of a taxpayer who enters into a written binding contract before December 15, 2017, to close on the purchase of a principal residence before January 1, 2018, and who purchases such residence before April 1, 2018, subclause (III) shall be applied by substituting ‘April 1, 2018’ for ‘December 15, 2017’
I like the part where it states, "and who purchases such residence before April 1, 2018." That sounds like a lot of leeway/grace. Does this help your client?