I guess this comes down to your risk tolerance.  We're talking CTC so due diligence is required, right?

IMO based on the facts presented, "the information provided by the taxpayer or a third party appears to be inconsistent" (if this sounds familiar that's because we just tripped Form 8867 #4).

I'm not one to jump in front of the bus and check "No" on #4a which asks "Did you make reasonable inquiries to determine the correct, complete, and consistent information?"  So I would make some reasonable inquiries, as dkh said, have them create an account and see what their account shows.  If they can't do that it might make me grumpy but then they're going to have to give me a list of bank deposits and dates of what they did receive.  If that's what I think they were supposed to receive based on prior year's income then I'm probably comfortable enough to take a tax position.  I have two data points that align.  I, of course, document this in my file and answer Yes to #4a and #4b on the 8867.

Rick