Firstly, genuinely well done & admiration for being a Foster Carer.
But, sorry, I don't doubt that your intentions may be good, but this whole thing sounds inappropriate to me - sharing info about a foster child with a stranger?? Why was she listening, and why were you discussing such personal info in a public place? Costa?? Public WiFi?
Is there no private place that you can take part in any meetings?? At home / in a library etc?
I admit that i have no experience of this.
My only suggestion is to speak to the organization who are 'in charge' of any fostering agreements with you. There are Fostering Support groups - I believe you really need to ask for professional advice, even anonymously, and no, please don't try to trace a random person who listened to your conversation (even if one sided) & may, or may not, be totally innocent.
Sadly these days you need to be so careful, as you are a person with such responsibility for vulnerable children, I can understand that you would like to have contact with another in the same situation - maybe write this one off, and rethink - however, my main point is that you say that you say you were in a 'rush', but then spent 2 hours speaking with a total stranger?? (She isn't a friend - yet - your DH is right in this case),
Good luck anyway, but those children that you foster must need to be protected. Random stranger may be lovely & caring - but may not be.