Thank you for your question. Mouth herpes (cold sores) have two types. Type 1 are often found above the waist and are commonly called cold sores or fever blisters. Type II are found below the waist are are often associated with genital herpes. Both types can affect the skin in any location though.
According to Medicine Net, the following are the ways that oral herpes are spread.
a fever, a cold, or the flu;
ultraviolet radiation (exposure to the sun);
changes in the immune system;
trauma to the skin; or
sometimes there is no apparent cause of the recurrence.
When the blisters are blister-like sores, they are contagious and can spread by kissing. You can can also spread herpes without the symptoms so you must be careful.
Here is a link to Medicine Net about how to treat oral herpes http://www.medicinenet.com/herpes_simplex_infections_non-genital/page6.htm.
You can certainly transmit mouth herpes into genital herpes by oral sex and she should certainly seek treatment prior to any sexual conduct. On the other hand, a person can transmit genital herpes into mouth herpes the same way. There are precautions that can be taken such as condoms.
I hope this helps.