I'm sure she's probably read the Nursing Mothers Companion. If she does have this book, have her refer to the section 'Survival Guide for the first 2 months' there's a section about fussiness and colic that's very helpful.
There are many reasons why this could be happening. If it is indeed something in her diet, dairy is the most common. Just make sure to tell her when you eliminate dairy to watch out for 'casein' or 'whey' that's in many foods. Those are related to dairy. Elimination diets have to be all inclusive, or they don't work, and they have to be done for several weeks before change is noticable as well. I had to eliminate caffiene from my diet because it bothered my son, where as it did not at all effect my daughter. And I'm not a big coffee drinker by anymeans!
I would definitely recommend the Nursing mother's companion though, it's got a ton of great information, and it's an excellent referral guide.