Here is what I would do:
Thank the friend and tell them you'd love to take them up on it, but want to be sure they know what they are getting into. Invite them over for a walk, with you.
Show them how to harness your guys (my mom once called when she was dogsitting, because Ginger's harness kept slipping off her but, lol, I have NO idea how she was putting it on her
), and go for a walk.
This should give you an idea how confident you feel in your friend and give your friend a good idea of what you expect.
Also, go over a few SOLID rules:
no interacting with other dogs (you can do that with them when you are home)
no letting them off lead
Good luck, and stop worrying.