'Biggest Loser' Trainer Dolvett Quince -- Super Skinny Champ Isn't Healthy ... Yet

"Biggest Loser" trainer Dolvett Quince still isn't happy with winner Rachel Frederickson's weight -- after she dropped from 260 to a scary 105 pounds -- but he tells TMZ, he's determined to get her healthy.

Rachel won Season 15 earlier this month ... but her dramatic weight loss shocked viewers, who claimed Rachel had lost TOO much weight, bordering on anorexic-looking.

And it appears Quince agrees -- he tells us, "We're going to get her healthy. She's going to be alright ... She's getting healthier every day."

Quince says he's still working with Rachel -- pro bono -- and has a plan to help her get better.