Say the boundary in operational terms
Name the contact, behavior, topic, access, or commitment you will and will not participate in. For a message about how to address an unequal friendship effort, use this factual base: use one or two recent examples and name the trust, effort, money, or inclusion issue they created.
Define any necessary exception
If an exception is necessary, give it one purpose, one channel, and a clear limit. Ask how your friend understood the situation and propose one change that can be tested. Make the immediate next step clear enough that the recipient can answer without reconstructing the entire history.
Respond to escalation safely
Do not assemble mutual friends, post indirect messages, or use confidential information as evidence. Stop direct contact and use appropriate support when there are threats, stalking, coercion, or safety concerns. You do not need the other person to agree with a boundary for you to follow it consistently.