These usually come when there are exotic investments that aren't required to issue 1099 by the Feb 15 date.  REMICs and the like.  Client probably doesn't know how her broker is investing her money, and broker probably doesn't know how this delays her tax filing.  Next year she will probably bring some K-1s.  Did the original return show a refund, and the amended will result in a balance due?  And she doesn't want to pay online, or is afraid the payment will not be matched with the return?  

No one is going to keypunch the amounts on the 1040-X either way, they are going to manually enter the adjustments in the format required by the IRS system after looking at the document you file.  1099's aren't attached to paper returns unless there is withholding.  Keep it simple.  "Additional interest was reported on 1099 received after original return was filed."