Can a lab jump a 6' fence?
Absolutely, they can (depending on training, age, weight and health of the dog)
Yellow lab jumping and scaling a 6' 3" fence right there...just for a ball.
Let's not forget that Labradors were specifically bred to retrieve fish and later bred and honed to retrieve game (mainly fowl!)
They can and will dig through almost anything to get what they want, as well. Rocks don't help. We didn't have chickens at the time, but in our old house (thankfully we have since moved) their dogs used to dig under the fence all the time to get to our little tiny dogs when we'd take them out for potty breaks (we never left them outside alone even for a second). We tried cinder blocks - they just dug and shuffled around them enough to get the dirt to settle a certain way so the blocks would fall in and they'd come right through. We tried pouring concrete and they just used the other side of their yard (not on our property so we had no control over it) to get out and jump our fence. We built a taller fence in the back - they jumped that as well. If they can't dig they'll jump and if they can't jump they'll dig, and to an untrained dog ... chickens are nothing more than mealtime.
We reported the dogs at least 10 times and several times they were actually confiscated by animal control - and every time the owners bailed them out of doggy jail.
I'm not saying this to scare you, I'm saying it to make you aware that (in my opinion) you may be in for a real battle and
I don't believe in candy-coating things when it concerns an animal's life (I candy-coat almost everything else, though lol)
OR...the dog may not give two hoots about your chickens and do absolutely nothing
I don't know what to tell you on this one (we ended up moving only for the safety of our own pets!) but good luck.
Never underestimate the power of a dog when it's made its mind up to do something.
Suggestion:
I would have another, lengthier and more serious conversation with the new neighbors. Be polite and courteous and respectful, but let them know that they should NOT take your politeness as passivity. You want good relations with your neighbors (everyone does), but they need to know that this is not a joke to you and you are seriously concerned about your chickens. It is a natural instinct for labs to go after chickens - that's what they were initially bred for.
Source: my own big mouth and not-so-humble opinion.
[Edited to add] IF the dog finds a way into your yard the first step is calling Animal Control...which you probably already know. IF he/she finds a way into your yard I would at least ask the owners if they would consider a dog run for their yard. I don't believe in tethering animals, it's inhumane...but a nice big dog run would prevent him from accessing your chickens in the future.