First, does the old MacBool Pro have a Firewire 800 port on it?
Secondly, is the amount of space used on the HDD less than the amount of space on the SSD?
Third, just to sure, you would want to replace everything on the SSD with everything from the HDD? That would include the Mac OS, correct? (I suspect the new MacBook Pro is running at least Yosemite (OS 10.10.5), whereas the old MacBook Pro is running with a different version of the Mac OS. Again, is that correct?)
Fourth, has she been making backups of the stuff on the old MacBook Pro, and preferably to an external device?
Last, besides Disk Utility, do either of you have a more robust disk cleanup/maintenance/repair program, like TechTool Pro or Disk Warrior? Also, has she been following any cleanup/maintenance plan on a consistent basis?
All of that is important in order for this project to be a success.