My friend has a 5 year old black lab who was rescued at 6 months. He didnt have the best start in life - he was kept in a cupboard under the stairs for most of it

He's known my dog since he was a pup and has always had an issue with him. Barking and chasing him...but not really in the sort of way that would make you think it was a fun thing. Nothing bad has ever happened. We've tried walking them together on and off lead, leaving them to it, intervening. It seems to be worse when it's my friend on her own - if her husband is there it doesnt happen so much (the lab is 'his dog' although they share the walking etc he did all the training so his word is law).
The lab doesnt have issues with the majority of dogs. We were both at a show at the weekend and of the hundreds there it was henry he picked out of the crowd to bark at. my friend thinks he might have issues with young dogs. also, he doesnt seem to play much - only with humans - i wonder if this is something to do with his start in life.
it's such a shame as every other month we stop over and the barking (and my friend's reaction to it - lots of shouting) makes it a stressful experience! if the lab comes over all aggressive (barking, standing very rigid, high fast moving tail wag) henry goes into injured puppy pathetic mode doing the whole submission thing, so i think he is doing his best to diffuse the lab.
any ideas what we could do to help them get on better?