I agree that it was a bizarre choice, but my understanding of it is so that we can witness and share SV's utter shock, and her realisation that it wasn't only her (SV's) life that resulted in her losing a significant part of herself; that this can happen to people who we least expect.
Had the character become, for example, a solicitor, it would have been more realistic, but the message would have been lost.
It highlighted that sometimes, when we make assumptions about people we are far off the mark, and sometimes, when we look at our own lives, and expect other people to have "perfect" lives, that we can be wrong.
Obviously this film was pre-social media, but I could see that the scenes with Joanna Lumley have parallels with Facebook, for example: how on the surface, everyone has these perfect lives, and yet scratch the surface and it's often far from what it seems.
I think it worked well, if you look at it through Shirley Valentine's eyes, rather than through a 2023 lense.