What is the best type of exercise for someone with lower back pain?

There really is “no best”, it all depends on the exact cause and a detailed account of the persons history
After receiving medical clearance, after all we want to make sure it really is lower back pain and not something else, and after physical therapy( ( if prescribed), it is then up to the trainer to abide by the exercise plan of the therapist and to keep in contact with them as the client progresses.