[throwaway568]

If you know them, they know you too, and they are sharing with you for a reason. But if you’re unsure how best to support them - ask this:

“Would you like me you think through potential risks and pitfalls, or are you looking for emotional support?”

If they answer the former, go right ahead and challenge them. If the latter then share your hopes for their success. Sometimes they know they have blind spots and they need your help before investing too much effort in the idea, sometimes they just want to share their enthusiasm.