Catching herpes from non human sources such as toilet seat etc is very rare. If the undergarment had been laundered then it is highly unlikely that transmission would be through this method.
Transmission can occur during sexual intercourse, or skin to skin contact. Transmission can occur even if the person with the disease has no visible sores. Also using condoms does not always protect as there is still skin to skin contact in the genital area.
Your daughter will need to get the appropriate treatment from her doctor or clinic, and will have to inform her previous intimate contacts they will need testing and treatment too if affected.
This website has more info:
http://www.cdc.gov/std/Herpes/STDFact-Herpes.htm#Howspread
In the meantime she will need to avoid intercourse or intimate contact with partners during period of outbreak.
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