You'll have a sobering influence on your friend's thought processes. In conversation, you'll encourage them not just to skim the surface of a subject or an idea, but to grasp its details and complexity. If they tend to zip from one idea to the next like a butterfly, never stopping for long at any one, you'll help them learn to focus and explore in a more purposeful way.
You'll also offer them sound advice for dealing with issues in their life. You'll help them to take life more seriously and to recognize which subjects and situations merit a more probing investigation. You can show them how to take their high-flying or abstract ideas and ground them in a workable, well-functioning framework, to make them more lasting as well as more accessible to the world around them.