My friend has (mouth) herpes but still lets people eat/drink after her. How can I get her to protect others?

Be aware. We do not need to be "germaphobes" but we can be kind to those around us. If your physician has diagnosed an infection in you of any kind , inquire how it is transmitted and how to prevent same. Truth be told some "terrible" bugs are real wimps in the real world. The common cold can stay on door knobs. Sexually transmitted diseases are only spread with close contact in a narrow temp & moisture...
Tell her the truth. You need to explain to your friend that she puts at risk those who eat from the same dishware when she has active lesions and that she needs to warn her friends/family members of the risk of infection associated with that.