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In Lightroom, you could try to paint in some sharpness on the pianist with the Adjustment brush. I have this challenge often, not so much in concert photography, but in other low light/high ISO conditions. My approach would be to go to Photoshop, and reduce the noise with Nik Dfine or other noise reduction application. I'd flatten the layers and then duplicate the new background layer and apply a High Pass filter to it (Filter/Other/High Pass) and switch the Blend Mode to Soft Light or Overlay, depending on how sharp you need the pianist to be. This layer will oversharpen the whole image, including the trumpet player, so I'd hide this layer behind a layer mask (hold down the Alt/Option key while clicking on the layer mask icon at the bottom of the Layers panel), switch the foreground color to white and paint over the pianist with a soft edge brush. You can then dial up or down the amount of sharpening with the Opacity slider on this layer. Flatten the file and you're done.
However, a more fundamental question relative to the image you posted is whether you really want to sharpen the pianist at all. Is the focal point of the shot the trumpet player? Or the duo? If it's the trumpet player, I'd leave it alone.