I feel like I should persuade my best friend to give up on adopting children
Why would you persuade them to give up on adoption?
It's bad for the environment, it's irrationally expensive, I'm also scared of them.
It is wrong to stop someone from adopting unwanted children. Kids need loving and supportive families, they are already born and not creating new people for resource competition.
I'm not stopping her from adopting, I just want her to feel bad about doing it.
She's doing a very kind thing. There's no reason for you to make her feel bad about it. You should really be supporting her instead.