M.H.
Tell her. I didn't have this exact situation but I was a naive first mom and didn't realize I didn't have enough milk to feed my little one. I was a petite woman so when I saw how little she was I just thought it was in the genes! After the doctor told me I was starving her, all my friends and even some of my family, said..".yes, I noticed!" Not only was it hurtful that my friends were uncomfortable being honest with me but my little one was suffering because of it. If you are truly friends she will take the information and investigate! Just be gentile....
M.