Firm on the standard, warm with the person. You can do both in the same sentence.

Warmth and rigour get treated as a trade, so people either soften the standard to protect the relationship or harden the tone to make the point land. Neither is necessary. Being clear about what has to change while making it obvious you are on their side is the combination that actually produces change, because nobody improves while defending themselves.

Say the specific thing that needs fixing, then say plainly that you think they can do it. Keep both parts short and avoid burying the standard inside compliments. It works better than either half alone.