My friend does not want her partner's daughter to be a bridesmaid. What can she do / say?

4 is too young to be a bridesmaid.

Flower girl, 4 is a good age.

Your friend did not say.
If her partner wants to have his / her daughter at the wedding, she could be on his side. Many parents have their children with them. When that happens, they have nothing to say.

I would not have a four-year-old bridesmaid too young.
I would not say anything.

I would tell the person that the person who marries her can leave her child beside her in any way he wants. It can go a long way, make the four-year-old a flower girl and then bridesmaid.