A glass of wine or a beer to help stimulate milk flow is going to have a negligible effect.
It’s benefits outweigh its potential harms.
I wasn’t aware once a mother was nursing that it became necessary to stimulate milk flow. How about smoking pot? Wouldn’t that help stimulate milk flow too?
You might want to speak with a lactation consultant before you pass this kind of advice. There is no safe amount of alcohol to consume that guarantees not to have an effect.