The overpowering need for political correctness in society is a laborious strain on expression and limits what people feel comfortable discussing. There is a social stigma around the descriptive word "fat" that is illogical.
There is no logical reason why people should be offended, or afraid to use the word "fat". It alone is not a derogatory word. It can be used to insinuate unattractiveness, however this assumption is on the part of the listener.
Society has given the word "fat" negative connotations, and while it is associated with an increased risk of a number of medical conditions, this doesn't seem to justify the stigma associated with it.
The condition of being fat, in most cases, is the result of eating too many calories and not getting enough exercise (thus, burning off calories); this is a simple fact. Thus, it is the person's own fault, and a result of their own decisions that they are in the condition they are in. Even in the rare circumstance that the obesity is caused by a legitimate medical condition, this doesn't nullify it's use to describe someone.
In general, people need to stop being so easily offended. Getting upset because someone used a descriptive word to describe a personal trait you don't like, is an issue on your part.