This is in very poor taste. The interpersonal details of what went on between Charles and Di have been out there for all to read, yes, but at this point do people really feel the need to judge the third (fourth, fifth, etc) parties involved? Obviously Camilla was more than a passing fancy, they're married now, and it's not up to this woman to "stand up" for Diana by snubbing Camilla due to her own moral judgements.
Declining quietly is one thing, but this just smacks of attention-grabbing.