That is almost a form of abuse to be honest. I hope your friend has learned to set boundaries and enforce them. People like her mom are very good at manipulating their children and seeing absolutely nothing wrong with it.
The underlying problem is the lack of boundaries. Moving away does not solve that problem. Only setting and enforcing the boundaries will solve the problem.
No amount of distance is a boundary. People can travel, use cell phones, or any other method to get past distance. It is only a logistical complication.
Psychological and emotional boundaries are required. Cutting off contact by itself is not enough. You have to communicate that you are cutting off contact. That's what establishes the boundary. In many cases you won't get a restraining order if you haven't established that boundary. The restraining order is a tool for physically enforcing an established psychological and emotional boundary.
I agree with you, seeing how even my spouse's 16h-drive-away mother is still managing to insert herself into her child's life several times a day. I can see on my wife's face she's been talking to her mother when I get home. Even with new recent boundaries set (we're not visiting anymore, no more answering the phone or WhatsApp 33 times a day and then when not answering right away having other family call to argue that you're abandoning your family, fuck all of them), and I feel my spouse has made tremendous strides there, the mum is still finding ways to do this.
Moving away solves the problem if you move far enough away that they can only visit rarely. You don’t have to answer the phone and text messages can be easily blocked.
Until they show up at your door, decide to move wherever you moved, or even try to move in with you.
You can't solve a boundary issue by not dealing with the boundary issue. At best, you can avoid it and hope it's no longer a problem. The underlying issue still remains.