Er . . . no offense but the information here is too vague to recommend any course of action . . . there are two issues.
First, what is the component being described, and what is the symptom that you're trying to fix? If it's an air-variable capacitor (cylindrical group of tiny plates moving in and out of each other), then they aren't silver (they're aluminum), and if it's scratchy when turned it's most likely caused by the copper grounding spring on the shaft, not the plates themselves (which shouldn't touch each other at all). I guess it could be a roller inductor . . . but these are usually found in transmitting equipment and antenna tuners, not receiving equipment.
Second, silver oxide (i.e. tarnish) is an excellent electrical conductor . . . this is one of the reasons why silver is the traditional plating material for mil-spec. RF connectors and contacts. So even if what you're talking about is a roller inductor or silver-plated contactor, the presence of tarnish doesn't mean that it's a likely source of whatever problem you're having.
The best thing to do would be to get the make and model of your friend's radio, as well as the problem that he/she is having with it, and post this information to one of the many internet forums that specialize in shortwave and amateur radio interests.
First, what is the component being described, and what is the symptom that you're trying to fix? If it's an air-variable capacitor (cylindrical group of tiny plates moving in and out of each other), then they aren't silver (they're aluminum), and if it's scratchy when turned it's most likely caused by the copper grounding spring on the shaft, not the plates themselves (which shouldn't touch each other at all). I guess it could be a roller inductor . . . but these are usually found in transmitting equipment and antenna tuners, not receiving equipment.
Second, silver oxide (i.e. tarnish) is an excellent electrical conductor . . . this is one of the reasons why silver is the traditional plating material for mil-spec. RF connectors and contacts. So even if what you're talking about is a roller inductor or silver-plated contactor, the presence of tarnish doesn't mean that it's a likely source of whatever problem you're having.
The best thing to do would be to get the make and model of your friend's radio, as well as the problem that he/she is having with it, and post this information to one of the many internet forums that specialize in shortwave and amateur radio interests.