What should I do my wife is being bullied?

My wife is being bullied by several women in her coffee morning group> And yes I do realise that the situation seems a trange one. The root of the problem seems to be my wifes small size (only 5-0 and roughly 95lb) her friends and other women in the group have picked up on this and are always digging and sometimes being very bitchy, this was having an effect on my wife as she would get very nervous and quiet before the morning in question or any event they all share. I thought this was bad enough and I hoped it would die down as my wife seems desperate to fit in with these women (the children attend the same schools ect)and she said she would handle things. But I arrived home a couple of weeks back and witnessed one woman actually sitting on my wifes stomach/chest area as she pinned her to the floor, every one else was having a good laugh and my wife was trying to make out she was, to save face in some way but her humiliation was obvious not to mention her pained expression as this bigger woman was obviously hurting her. I have since found out that this is not an isolated incident and has happened a lot and is supposed to be funny, it is affecting my wife badly but she has pleaded with me not to say anything to any of her so called friends. I’m at a loss as to what to do?

Dave.