Getting your first clients in any business always seems to be a challenge. This question comes up time and time again with the startups I work with. Depending on the type of consulting business you friend is starting, they may be able to look at their own internal network, including friends and family. If they are working in a niche sector, I would suggest they approach any regulating bodies, chambers and forums that have an audience of the right type of client.
Getting your first few clients can take time and it can be frustrating. If you or your friend would like to book a call with to ask any question or talk about a strategy, feel free to. I'd love to hear more about this new venture.