I read a really good article recently, aimed at stopping a dog from chasing. Written by a former Police Officer it made a lot of sense, but.....
Citronella collars were the ultimate in last resort and cannot be used just as a collar. His method (assuming the previous suggestions had failed) was to rub raw lemon into the gums of the dog, leaving the lemon pith (not zest) for as long as possible - this involved two people holding the dog and forcing the lemon into its mouth. It will be an unbearable amount of flavour scent and one the dog will not want to repeat. As I said before, this is (in his opinion) the last resort. The collar is then used, sparingly, to stop a behaviour by reinforcing the scent at at time a command is given. The principle is that the horrible taste / smell is not worth the self reward of chasing.
My view on this? The collar has a value and purpose but only when every other avenue/method has been exhausted and that it must be used properly. To use a citronella collar without putting the effort in is not only irresponsible but very likely to induce long term behavioural issues not to mention downright cruel on the dog. As Bizzylizzy points out a dogs nose (and taste buds) are thousands of times stronger/better than ours and would you like pungent lemon scent forced up your nose?
Leave it to the professionals and put in the hard work and effort to train the dog properly. Coral eats poo. She does it for attention and especially at my house where the garden is smaller. At OHs we don't give her the opportunity to do it, it's cleared up immediately and she has space to burn off energy.
I would suggest your friend knows nothing about training a dog and is just plain lazy - I doubt a vet would have recommended it, more likely a discussion took place and it was an option. Your friend ought to seek professional help from a positive trainer who can help with a "leave it" command - it's a viscious circle: dog eats poo, gets worms, eats more poo as it's hungry. A good quality food, possibly with a mineral supplement and training would be a better solution in this instance