Radical Candor (Kim Scott) has two axes: how much you care personally about the person, and how directly you challenge them. The goal is to do both at the same time.
Get clear on the real issue before you open your mouth. Is this a skills gap, a behaviour issue, a workload problem, or something personal going on for this person? Is it a pattern or a one-off?
State specifically what you saw or heard. Facts, dates and examples only. No interpretation, no assumptions about intent.
Explain what this has cost the client, the team or the business. Make the stakes real and specific so they understand why this matters.
Ask them what they think should happen before you offer your view. Let them co-create the path forward. Always document the agreed next steps in writing after the conversation.