I think you only rely upon substantial factor when each D isn't a clear but-for cause, e.g. two fires combining. From that fact pattern, it sounds like each is just a but-for cause, making them all jointly and severally liable, unless you can get them off on proximate cause.
If dr. had warned, decedent wouldn't have swam.
If lifeguard had been present, decedent wouldn't have drowned (lack of lifeguard substantially increases rsik of drowning, thus it can be viewed as a but-for cause)
If friend had stopped him, decedent wouldn't have drowned.
Of course, each of the above have counter arguments, like did friend or dr. have a duty to warn him? Did the pool owner have a duty to have a lifeguard present? Would decedent have listened to friend or dr if warned?
Please, someone correct me if any of this sounds wrong.
all 3 of the defendant's conduct were but-for causes of the plaintiff's injury. The next step is determining whether any of the defendant's conduct was a proximate cause of the plaintiff's injury. The substantial factor test is simply one way of determining proximate cause. The substantial factor test has nothing to do with whether the conduct was a but-for cause.