I agree with Jennifer P - don't blame someone unless you KNOW they gave it to your child. Try not to focus on the anger, focus on preventing outbreaks and contaminating others.
My 1st child got "lip sores" when she was about 3. I have no idea where she got it from but the pediatrician was very clear it's nearly impossible to prevent with little kids...esp because they often have their hands in their mouths and/or don't wash their hands enough. She said it's just "luck" if someone doesn't catch it in childhood.
Yes, it sucks that there's no cure, but that doesn't mean that she has to suffer with ugly, oozing or crusty sores that the whole world will see. There's a lot out on the market now to help. Lysine is one. L-Lysine supplements is another - to prevent breakouts too. And there's a prescription for Acyclovir if she's dealing with an illness, since a weaken immune system can trigger an outbreak.
Also, as Jennifer said, if you educate yourself and your daughter now, you will know what foods to avoid/limit, and how to immediately react when she feels the "tingling" of a sore coming on.
My now 7 yr old almost never gets an open sore now. We know how to deal with "the tingling" before it turns into anything more. We keep Lysine and Abreva lip creams in our medicine cabinet.
FYI - as you know, she can now pass the virus along to others. Make sure she keeps her hands away from her mouth when she's "tingling" or fighting off an infection. This can easily be spread to siblings or parents and even turn into "eye herpes" in someone else. The lip and eye are the same strain. The private parts herpes is a different virus. Supposedly once you get it, you've got it. Period. And it won't move from lips to eyes. However, the eye one is particularly difficult to deal with as it can go into the eye and cause other complications.