I think that may be a little harsh given how little information we have here. .................. It's not like the solicitor is in a position to verify that the bank will release the mortgage until the funds are sitting in their account.
The solicitor cannot verify that the funds will be there to complete the purchase. But this would be my concern:
They had previously cried poverty when the 10% non-refundable deposit was due and my solicitor agreed it was unusual but advised me to accept they would pay the full amount on the closing date.
Unless there is more to this story than we have heard so far.