I think “encourage” is such a beautiful word. Encouraging a friend comes from such a positive place that I cannot imagine a friend would receive the encouragement in an ungracious manner.
If it is a true friend, somehow I feel the friend would welcome encouragement. If might be what he/she is waiting for.
I would be honest with my friend and offer encouragement and go as far as to assist her in changing whatever the cause of his/her weight gain is attributed to.