Welcome to the site!!
I am 28, however, my IBS started in my teens as well. The best thing for managing is to eat well and never eat right before going to bed. I also did a lot of Imodium popping when I wanted a good social life. Sadly, it is just something you have to get used to and come to terms with.
Probiotics are good bacteria your gut needs to function properly. Everyone needs them. It would be SUPER beneficial for your daughter to start taking them. I take them twice a day every day. I will be taking them indefinitely. Here is a good link for you to acquire some knowledge: US Probiotics
I take the supplements (Culturelle & Kyo-Dophilus) and I also eat Dannon Activia yogurt. Activia is loaded with probiotics AND it is proven to help your gut function in 2 weeks (if eaten every day for the 2 weeks). I also think the Activia is great because it has calcium which helps soothe any inflammation and helps control D.
Probiotics are essential for healthy intestinal function. As far as I know, you cannot OD on them... the more, the merrier. However, when taking the supplements you have to be prepared for some added gas at first. Just like fiber, they should be introduced slowly. I started taking 1 pill with 1 billion live cultures a day and then after a couple weeks bumped up to 2 billion, and so on. Now I'm up to about 5 billion a day.
Please, don't be shy to ask anymore questions or let us know if you have any other concerns. I wish you and your daughter well.