Has anyone ever heard of chocolate labs having trouble with their knees?

Not labs specifically, but many dog breeds have chronic knee issues. I know from experience that basset hounds, maltese, havanese, westies, and yorkies do.

Most heavyset (not necessarily fat, but thick and muscular) active breeds can be prone to all sorts of joint issues, labs especially, due to their popularity and the unfortunate lack of discretion in breeding for profit. If these dogs were from a reputable breeder, they should be notified.
Also, the owners should have a serious consultation with the vet about what sorts of activities could more likely lead to these problems.

I had a black lab who threw out his knee and we were told it would happen again. He just jumped down the stairs of our deck… nothing crazy. He was about a year and a half old. Super duper expensive (we ended up finding a new owner for him).