I bet everyone has been in this situation, your dog is on the lead and a dog comes bounding up to you and your dog, the owner says "shes friendly" and whether or not your dog is ok with other dogs its still not right, because the dogs could end up in a fight as feeling threatened. The underlying rule which every dog owner should know that when your dog is off the lead and you see a dog on the lead you should put your dog on the lead, but some people don't know that or ignore it.
What most people should know that when you see a dog on the lead that dog maybe is reactive to other dogs , old, in season or has an injury. There is the yellow band dogs can wear which means you should keep away as the dog needs space, and you should not go near this dog, also some dogs wear a collar that says nervous or something similar and that's when you should keep away and give them a wide berth as well.
I've had countless experiences when walking my dogs that dogs come up to my dogs and sandy has a go at them (warning growl), usually they are entire boys which she hates. Sandy is a peoples dog which means she goes up to the person first then the dog and Murphy is a dogs dog which means he goes up to the dog first. So usually Murphy is always off the lead on our walks as his recall is good and he always follows and Sandy is on the lead because she likes to go into bushes and so I only let her off at specific places, but always when I see dogs coming towards me who are on the lead I put them both back on and walk quickly past. I've had experiences when both my dogs are on the lead and a dog follows us and the owner is oblivious, that drives me mad.
So the moral of the story is if you ever see a dog on the lead or with a yellow collar on put your dogs on the lead and give them a wide berth. And it is never ok for a off leashed dog to run up to a on leashed dog.