If she wants to make friends, she has to put herself out there. If she doesn’t want to do that then, kindly, there isn’t much you can do.
Depending on where you live, there must be clubs and activities that interest her that she could join - crochet, padel, choir, running, am dram, women’s institute, volunteering, orchestra, something? Where I am, I see a lot of groups springing up for young women looking to make friends. Mostly book clubs and walking groups. How to make friends is a common topic on the Reddit forum for my city so she might get specific ideas asking there.
It’s scary walking in somewhere the first time but you could help her with some tactics for pushing past that feeling eg handy conversation starters and remembering that people are generally nice. And if she hates it, it’ll be over in an hour or so anyway.
The key is to find the right place and keep showing up consistently. She won’t meet her new best friend on day one but even just being part of a group and exchanging a few pleasantries helps fulfil a need for a social life. It will also make her a more interesting person so she’ll have more to talk to friends about in future.
If in person meet ups are really too far out her comfort zone, think about online alternatives. Where I live, there are a few podcasts presented by local women and their fans chat on Patreon and organise meetups. Another online route to try is Bumble for friends.
Maybe you could encourage her to try one new thing a month and see if she likes it?