I think on an individual level your friend is just making a decision to offer up a story which means no debate or potential criticism. I can't blame anyone for wanting to do that.
However - on a wider level, the 'not enough milk' thing is bloody damaging and dangerous to mothers trying to bf. Let me clear, so I don't offend - yes, A FEW women will have trouble with under supply and need to supplement or switch to formula. Tubular breasts, severe anaemia etc - it can happen.
But for the majority of women thinking they don't have enough milk, it just isn't true. They are usually trying to feed in the 'wrong' way or with the wrong expectations - expecting a new born to go hours between feeds and then worrying they are too hungry when they want to feed again - expecting a bf baby not to feed little and often, but in big 'doses' like most ff babies are fed - all of the mistakes mothers make when they and people around them have limited knowledge and experience of bf-ing.
If you are a new mum bf-ing, and finding it tough, and most of the mums you know tried and gave up bf-ing, and most of them said it was 'not enough milk' - well, you know where that's going. Thus the myth carries on.
I do feel as well for those women who do genuinely not have enough milk. Women using it as a line aren't helping them.
btw, as someone who is pro bf and bf-ed herself, I think there are plenty of reasons why someone doesn't bf, social as well as emotional and physical - and that's fine. BF-ing is great but not when it makes a mother deeply unhappy, or is very hard under social circumstances she can't change.