Should my step daughter tell her partner she has hep b before having any type of a sexual relationship with them?

Yes! You can get hepatitis b through sex. She has to tell him.

Index | CDC Viral Hepatitis (http://www.cdc.gov/hepatitis/)

Yes, she should tell him

Let me ask you one question, if YOU had sex with someone who had HepB, and they never told YOU, would YOU be upset?

It's very selfish, and dangerous, to have a disease, and try to keep it away from your potential partner.

People have been sued, and lost, cases like this one.

I think you know the moral answer to this question.