My personal preference is WAIT!!! It's an awkward situation and you (and a lot of people I've met) are sincerely trying to help and not trying to mock them but the best is letting them finish. It can also make them feel more self-conscious about it (that's how I feel when it happens to me). I have noticed that my 5 year old nephew will patiently wait for me to fight through my stutter and I appreciate that, especially since he's generally a brat. I've also noticed that my 11 month old son takes notice of it because it's different than everyone else's speech. To add, like the commenters before me said, you did handle the situation very well from my perspective.