This mom needs help. It sounds like she has an eating disorder that she's projecting onto her son. If she doesn't get help for her own benefit, she needs to get it for his. It also sounds like she hasn't gotten even basic infant nutrition education. A child under 12 mos. should NOT be having any kind of cow's milk, reduced fat or not, unless it's milk-based formula. And juice is actually really FATTENING for kids that age. Given her concerns, it's the last thing she wants to be giving him.
My SIL had a lot of mental health issues following the birth of her son, and she withheld needed nutrition from him. My nephew, who is now 3, is a wonderful little boy, but he has a lot of behavior problems around food. He will NOT sit at the table. If you strap him into a booster seat, he'll start throwing his food on the floor. This is causing problems at his preschool, since it starts the other kids doing the same thing. (The court eventually took custody away from my SIL, in part based on this feeding issue; your friend should know that.)
What you may want to mention in your friend's case, though, is that withholding nutrition can often cause the opposite reaction. Children can very easily develop a starvation mentality and become lifelong compulsive overeaters. Since that sounds like a nightmare scenario for your friend, I actually really recommend mentioning it to her. She needs to know that baby/toddlerhood is the time when we learn to recognize our satiation points. If she doesn't give her son a chance to stop when he's full NOW, he'll never learn how to do so, and he may overeat to extremes for the rest of his life.
In terms of calling CPS, I agree that that may be necessary, but it's also the "nuclear option." Before you do that, is it possible for you to work within your own network? Is there a pastor/priest/rabbi/imam/etc. who might be able to talk to your friend? If not, can you work with other mutual friends to stage an "intervention"? You may want to consult with a therapist who deals with eating disorders first. But if those options don't work, or aren't possible, then, yeah. I'd call CPS.