Answer: I would pull them aside and see if they are ok. I would address my concerns with them not eating and talk with them on how concerning that is to me. I might start talking with them by saying "I noticed your not eating as much. Is everything ok?" or "Im really concerned about you cause I've noticed your not eating. Im here for you if you want to talk." I would then refer them to a parent or Councillor that would be able to help them more. I would let the parent or counselor bring up them being underweight because that's not my place.
(Be careful with) If they need more support than that: I would offer to help them look up resources and invite them over to my house. I would let them know Im here for them and reassure them that others have the same issues.