Different circumstances need different types of support.

Sometimes, just listening so the other can vent. “Tell me more.” “And then what happened?”
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Sometimes, it is validation that they seek. “What you are feeling makes so much sense.”
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Sometimes, they do want a little pushback so they can really work through how much they want [whatever]. “What would happen if you took another route?” “Have you considered the tradeoffs involved if you followed this path?”
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Sometimes, they want you to stop them from actually going through with their plan. “As someone who really cares about you (and can see this a little more objectively), I really don’t think this is a good way to deal with the situation.”
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Sometimes, they want you to help them think through some options. “From my vantage point, I see a few options. I think it is worth exploring them before you decide what you are going to do.”
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Sometimes, they just need a hug or ice cream or some free time to take a walk alone. Or … Or … Or …
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There are so many ways to be supportive. Sometimes it takes true wisdom to know how to best support someone. Often, you just have to ask “What is the best way I can support you right now.”