With slip leads or choke chains it very much depends on how you use them, so many people just don't use them safely, and many of the more highly knowledgeable and experienced believe there isn't a safe way. Some people like them some don't. However many show dogs seem to use choke chains for the control and elegant simplicity.
If you watch people out walking their dogs it is interesting. Many people do just let the dogs pull and I guess they think that's what dogs do. But whatever the reason many of these people have no ides about the different types of collars and frequently the pulling dog has a choke chain, frequently the choke chain is even placed upside down on the dog to prevent it unlocking. It is equal to waking a dog with a noose round their neck.
The other thing you see is there same but every now and then the owner gets irritated and they go severe backwards yank yank yank with the lead. Not the quick sideways jerk release correction you see shown on TV. Because it is virtually a noose ( which are used historically for executions ) then incorrect and harsh or careless use of the choke chain can cause damage to the dog. And this is why people do not like them, and some training affiliations ban their use so a trainer permitting the use of these in their class would basically get struck off.
However incorrect harsh or careless use of the headcollar can also cause damage to a dog. Watch people using these and many are incorrectly fitted and incorrectly used. I guess all training tools need ro be used under the correct instruction.
With the chain as far as I can work out it is a short sharp shock so one or two corrections are supposed to be enough to gain a compliant dog, after that the correction only needs to be the sound of the chain. You can use them with just the sound of the chain and not give a full choke type correction which is in effect using the chain like training discs as a distraction to gain attention.
For your friend a half check might be a good compromise. I have some for my dogs loose fit as far as their neck goes tight fit as far as their heads go! lol! But you can have them fitted so as to hive a pinch correction if you like. I like them because they are easy to slip on and off the dog and are great for people who like that bit of bling. To be honest I think a lot of people who like the chains like them for their simpleness and convenience and traditional appearance... they simply look very 'dog'. Practical, hard wearing, easy to clean and not actually used as cruel torture contraptions.
I think it is a lot easier to train a dog using a correction tool when you are thinking of stopping unwanted behavior but in gaining required behaviours I'm a bit lost as to how they work? How do you train a dog using no reward? I genuinely am interested to now because I wasn't around dogs before positive training evolved and I know some trainers are strongly against using food/toy rewards in training. Any insight would be interestig.
and another choke chain trend I've seen is to have so much chain wrapped around the dogs neck the idea must be to stop them moving by the sheer weight of the metal. It's like that game buckaroo were you hang as much as you can on the mule.