Wow, this is a hard question! If this is someone you care about, there are ways to protect yourself from getting the STD. If the STD can be treated with medication and cured, she needs to seek treatment. If she has a viral infection, such as HPV, Herpes or HIV, using a latex condom every time you have sex will protect you. If you want this relationship, both of you might consider going together to see a care provider and get all of your questions answered.
If this is a casual sex partner, then you would still need to use a latex condom or you might just decide that the risk of getting a STD was not worth having casual sex with this particular woman.