That post changes the nature of your original one. That reads like she (and maybe you) want what is called a "clean install" which- basically- means re-starting her Mac usage from scratch with no files, etc from the old one until she wants/needs them.
If so, she can attach a drive and copy all files from old Mac to it. Then store that drive somewhere. Start using the new Mac and download apps as needed (for clean app installs) and plug that hard drive in whenever she wants some file that was "on the old Mac" to move to the new Mac.
Eventually, there will be no more old files to move because she will have pulled all the ones she actually wants over to the new one one-by-one. However, if there comes a time where she DOES recall another she wants, the hard drive and/or old Mac is always there ready to provide the file.
Unfortunately, there is no magic "migrate only the non-junk stuff we don't want onto the new Mac" tool. It takes her (perhaps with some help from you) to figure out the keepers from the junk. There's no way for any app to be able to tell what she/you considers junk vs. "the good stuff" to be kept.
OPTION #2: To still use the first recommendation of Time Machine, she can spend time working through all of her documents/photos/music/apps/etc on the old one and trashing all of what she considers junk. When the old one is purged down to ONLY the "good stuff," you could run the same Time Machine process previously described to auto-migrate what is left.
This would still move some behind-the-scenes junk not easily accessed by someone "cleaning house" in this way but that useless clutter would likely be relatively small if she does a good job trashing all junk she can easily access.
If so, first clean up the old Mac to as close to how she wants the new Mac as possible, then execute the TM approach to migrate what is left to the new Mac.
Between BOTH options, clean install is usually favored. However, option #2 will avoid LOTS of "need the hard drive again" scenarios as she wants access to things that are still on the old Mac but not the new one (yet). So net is usually the #2 option vs. having to fetch that drive over and over and over again to get another file still "back there."
The sole benefit of option #1 is that it is an absolutely clean Mac with no junk clutter behind the scenes anywhere because you are putting everything back one thing at a time and thus only installing stuff that you judge as not junk at each of those moments.
The analogy here is a classic: moving. If you've lived in a home for a good number of years, you have a mix of stuff to definitely take with you when you move and clutter/junk you might trash. You could:
- take everything to the new house and then try to purge the clutter/junk there,
- purge the clutter/junk at the existing house first and then take only the keeper stuff to the new house,
- just move without selling the old house, leave EVERYTHING back at the old house and only go get stuff from the former home when you decide you definitely want it at the new one.
That last one is a pure "clean install" analogue: absolutely
NOTHING makes it to the new house without fresh scrutiny and a clear decision. The first one is using Time Machine right now with no purging:
EVERYTHING is going to the new home and we'll try to purge the junk there. The middle bullet is the "hybrid" option to get rid of the obvious junk before reaching a point where the rest will just move to the new home (potentially bringing some junk with you that you missed).
