A Mac that begins to run slowly is a common complaint. There are tools we can use to discover what is causing that.
Please have your friend use her browser to visit EtreCheck.com and download the free version of that utility. Have her install the application and be sure to enable full disk access when she does. Then run the application to create a report of the Mac's hardware and the software installed. Once she's run the app and created her report please post it with a reply to this message.
She must upload the full report. To see how, please click > How to use Add Text when posting… EtreCheck Report.
We’ll use her EtreCheck report to look for the things that may be causing her problem and advise how to correct them.
You can rest assured there will be no personal info attached to the report.
Meanwhile, removing the data from her internal drive is the LAST thing to be considered at this point.
The FIRST thing to do is use an external drive to create a Time Machine backup of her computer. It's critical that she backup her data before proceeding to any other step beyond the EtreCheck report. Having no backup just about guarantees that she will lose her data.
After we get a look at the EtreCheck report, we can provide the best guidance toward correcting the slow down. It may be that her best option would be to install the macOS onto an external drive, along with her data, and then use the ext drive as the regular startup, as you suggest in your post.
It might also be that the slow down is caused by some malware, and removing that or other conflicting software would correct the problem.
For now, have her focus on sharing the EtreCheck report with us.
