Science may not be able to prove ethical principles, but that doesn't mean that science has nothing to do with providing support for ethical judgments and ethical principles. For example:

• Science can sometimes be used to show that a given action complies with or violates a given moral standard.
• Science can often be used to estimate the consequences of an action. As long as we believe that consequences are important for morally evaluating our actions, it follows that science can be quite helpful.
• Science can often tell us whether a given moral goal can or cannot be achieved using certain means. Or it may be able to tell us which means is the most efficient.

My point is that science can be quite helpful in many situations even if it can't 'prove' our principles or establish moral goals and ideals. There is no reason for an ethical relativist to deny this.