How to handle when you meet a potential client in a public place?

I’m a coach. I was sitting outside at a cafe today editing copy, when I heard a woman at the table next to me, chat to her friends/co-workers? (2 guys and another gal) about how she wanted to stop drinking.

She mentioned it several times.

I found this a perfect opportunity if she were alone, but wasn’t sure of how to do that in public with what could have been a private conversation with friends/close acquaintances.

Any tips on how to responsibly deal the next time?