Your dog must have a clear understanding of the behavior you are looking for. In this case, a Sit or Sit-Stay is an appropriate alternate behavior. Be sure that all new people greet the dog calmly so they do not encourage the jumping behavior.
It is critical that the owners do not allow the dog to jump on guests at all.
If the dog continues to be persistent, it might be wise to use a mat to create a greeting spot or "place" near a door handle where you can direct the dog to go so the dog can be tethered with a leash but that is visible from the door so the dog cannot have access to visitors coming in.
Keep in mind that using this exercise will help the dog greet visitors calmly, but the dog may not generalize the behavior as it has only performed in a single location. The owner should now be encouraged to practice with the dog in many locations where guests are likely to be encountered.
If the dog is extremely persistent, you may find that stepping on the leash while it is on the floor and short enough to prevent the jump from the beginning is the best way to go.
Consequences for Jumping
One of the most effective corrections for problem jumping is to remove the very reward the dog is trying to obtain -- attention. To do this, remove any opportunity the dog has for physical contact or eye contact by having the guests turn away from the dog and tuck their hands underneath their arms when the dog jumps.