I've just started running with a guy at work, he's not very fit and hasnt run at all over a year. He has been complaining that he gets the shits after about 20mins and has to make a sprint for the toilet!!!

Is this likely to be caused my lack of core stability??? his diet seems OKish, and it's not something I've suffered from before so I'm out of ideas. Any thoughts? He wasnts to do a tri next year and that could prove messy!!!!