Is there any way you could walk your dog with the Lab and their owner regularly? Perhaps try and bore them out of their obsession with each other?
Another method could be to try and make yourself far more interesting when out on walks, so Indi looks to you for all things exciting, rather than the Lab. Short, exciting games like chase the ball, tug, hide and seek etc.
If you do think there is a real worry that Indi might take off after the Lab (or any other dog) have a light, long training line on her so if she does make a run for it, you can grab the end. Also try to be aiming to teach her a really solid recall.