I have a hard time believing that.My doctor brings up a 5 pound weight gain for me but a friend of mine is 100 pounds overweight and he doesn't say a word to her (same doctor). I know he doesn't say anything to her because when I talk to her about my appointment and tell her that he commented on the weight gain she tells me her weight is never discussed. Definitely, hit or miss with doctors
It is much more plausible that your friend is embarrassed and doesn't want to admit the doctor talked to her about her weight. Especially to a friend that just told her the same doctor told her to put on 5 pounds.
To admit to somebody that the doctor has discussed your weight with you would be very difficult to somebody struggling with their weight. It can make you feel very ashamed to have to admit that.
I know if a friend asked me whether the doctor had discussed my weight, I would find it way too personal to discuss and just brush the question off with "nope, we never discussed it" when we had actually discussed it at length.
Because to tell somebody that the doctor prescribed it, could make somebody think they have license to uphold the doctor's wishes and try to be involved in their weight loss.