Yes, sanding will smooth out the basebaords. There is a chance that he is picking up junk from near the bottom of the baseboard and this is making the trim rough. Also, the paint could have bits of junk in it already. Paint gets this way when it sits for some time or it just naturally gets snotty while working with it for several days. You can fix this by straining with a paint strainer or some old nylons. Also, using a good brush helps with keeping the brush strokes to a minimum, but also good technique is important. If you dry brush the paint on or overbrush the paint, the brush marks will show up more. The key to painting without the brush marks is laying the paint on fairly heavy so that it can dry down into itself and the brush marks will dissipate. You just don't want to apply the paint so heavy that it sags though.

thanks for the info Paintguy, I'll try that ..