Try and imagine her as an image of yourself rather than what she actually looks like, which oftern clouds our judgment, and ask yourself again if you are comfortable with the level of care the children are receiving from her.
How does this compare to the potential of what could happen if they were taken away from her - care homes etc. Would it balance off against that?
The social services would probably have looked closely into how this woman lives her life before allowing her to adopt (I know as my cousins have done it) so she her as a fit candidate and surely would have reviewed the situation before looking into placing another child with her.
As for her weight I guess that's down to the social services to investigate further.
However, if you feel the children are coming to harm then that's another matter. How about approaching her first with what you saw regarding her friend? How would she react to that do you think?
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My uncle once punched a man so hard his legs turned into trombones