A boater is not required to tow another boater. However, a boater is required to "render assistance" to another boater in distress. "Render assistance" means to provide whatever assistance one can provide without putting your lives or your boat in peril. "Render assistance" can take on many different meanings depending on the circumstances. It may mean calling a towboat for them, it may mean simply standing by until the proper help arrives.
Towing a boat can be dangerous and towing can damage either boat or the passengers. I've done it several times over the years but only after carefully considering the situation.