Everyone's advice here is on point. If you do use an XP machine, it should not be online, and email should preferably accessed on another device.
XP support and security will be non existent, you are essentially running a grandfathered computer at that point. And it should be used as such or completely avoided where possible. Linux Mint is a solution I was not familiar with but that sounds like an option to test on the newer old laptop that is being given to her mother. Or like everyone else is saying, VM is the way to go to try and run old software.
In a situation where the user is wanting this because they can't/won't learn new software, the next best thing is to keep it simple.
I would suggest to find a overstock warehouse that sells old computers, laptops tend to fail more, need more maintenance with batteries, chargers, and that's the downside with purchasing an old laptop. If a small desktop will do the trick with a monitor, that should last the test of time a little better. If not, find an XP compatible laptop and install it on that.
Older Dell latitude dual core laptops can often be found for $100 or less. Just keep it offline, run her favorite learning programs on that and make it dedicated to just that, turn off all internet ports, and a newer laptop would be preferable for browsing the internet if she ever decides to.