Along w/Karen's suggestions, here are a few things to try:

At 50 pounds, I am not sure how you would do this. When I first took my puppy mill rescue to my groomer, she had me pick the dog up and hand her to the roomer, BACKWARDS, so the dog is facing me. This is supposed to indicate to the dog that this person is OK. (My groomer has a PhD ABD in animal behavior: the human - animal bond). So when you want to dog to go with someone else, you would pick the dog up and hand the dog to the person, with the dog facing you.

A few other ideas:

When someone else is taking Chesney out, they need to have treats with them. I keep a baggie in my pocket on walks, with small pieces of hot dog. You can also use boiled chicken breast. For the start, this needs to be a "high value" treat, something the dog just LOVES and goes crazy for! They can give a command - come! - and treat. This works because "come" is an important command and one which should always be rewarded with a treat. Other commands, over time the dog can progress to praise as reward; but "come" is a treat reward for life, because you may need to use it to save the dog's life. So you need it to be re-enforced.

They can also use "walkies" or "with me." These are commands I use often during a walk, to get the dog moving after he has stopped to sniff something, to get back in line with me if he has wandered off to the side to check something. So again, these are commands the dog is going to learn and need right along, may as well start using them! These get the high value treat as reward (along with praise) until they are learned and the treat can become more intermittent.

Finally, having a dog slip it's collar can be a huge safety concern. You may want to consider using a harness. For my puppy mill rescue, she was able to slip out of so many, I found this one was "escape" proof. So you may want to try this.
http://www.ropengo.com/