The path of least resistance is to upgrade the SSD as you are doing, and possibly upgrade the ram as best possible for this mobo. Those two alone should speed things up considerably. Don't mess with CPU or mobo upgrades.
Next is to review the OS & software, as I'd guess it is pretty well bloated by now. You'll want to uninstall everything no longer needed, remove all update remnants left lying around by the OS patch process, and so on. ccleaner or similar to help decrapify it.
Otherwise, if more power is needed, and when isn't it ... I'd clone the OS to a vm. The customer will always be able to run everything just as it was, forever (sounds like she is resistant to change). This gives you the chance to give her a modern system, with a minimal Win10 on it, and virtualbox, and then you can present her with an auto-starting vm of her Win7 build. The only possible gotcha I can think of here is if any of her software relies upon graphics cards, which don't pass thru in virtualbox ... most office software runs fine in VM's, but you'll need to check her graphics design apps.
If there is a GPU requirement, then perhaps the path is towards Win10 (pro) with Hyper-V, and there you'll need to sort whether or not Hyper-V can pass thru to one VM, which is her Win7 environment. I've heard it can, but I tend to use VBox and others, so don't know the details of Hyper-V ...
hope this helps ...