S.B.
"Do this or I won't like you".
The perfect comeback, that even a child can understand and is appropriate is, "Fine, you don't have to like me. I'm still not doing what you say."
Being a mother and doing daycare for a long time, this "I won't be your friend" stuff happens. It's not that abnormal although it isn't nice at all.
You just have to make sure your kids know that friends don't be friends on an "only if" basis.
If someone says, "I will only be your friend if you give me your bracelet", it's perfectly okay for a child to know it's fine to say, "If you don't want to be friends, that's fine with me. I'm not giving you my bracelet."
I experienced it much more with girls trying that with my daughter, but boys can do it to. My son had kids who tried it.
I just taught my kids that if someone said that to them, that's not what "friends" say to each other and to call their bluff.
Fine. We don't have to be friends then.
It's not rude. It's a way for a kid to stand up for themself and it's done without even involving the other parent because heaven knows, there are lots of little cherubs running around this planet whose parents would never believe they would say such a thing.
Deal with it on the level of your own kid. That worked miracles for me.
Kids may say stuff like that, but it has no power if your kid is strong enough to say, "Okay. If those are your terms...see ya!"
Just my opinion.