as Janet said, this is not uncommon. I have seen it more often with ‘friends’ who feel threatened by the success of one in their circle.
I often ask clients about social habits and suggest that the ‘girls’ night out’ could be replaced by a fun activity that does not revolve around eating. I also try to prepare a client for this occurrence. When it comes from a partner, it is a very difficult situation but social interactions can be altered more easily.
Discussing strategies with clients in preparation for events like that is often helpful. Workplace temptations can also be anticipated (eg. the candy jar on the colleague’s desk).
It also helps to discuss with your client that the newly acquired healthy behavior does not need to be broadcast in a proselytizing manner and thus inviting such a backlash.