I am sure some others will chime in but I began my weathering adventure right here on the forum. Have your friend check out the scenic showcase and weekend photo fun threads. You could learn a lot from what the guys post. They are some of the best in the business. Next I would advise him to go out a take pictures of actual trains, buildings or vehicles he wants to weather. Advise him to take note of stuf like road grime, the color of the wheels and trucks, areas of water run off, rust etc.
Some of the items in my weathering tool box include. sandpaper, oil and acrylic paints, paint pens, brown stamp pads, rit dye and alcohol mix,spray paint, Bragdons weathering powders and Testors Dull cote. Notice no air brush.
The biggest obstacle I see is that people are afraid of taking that inital step to do it. Tell him to start with something small and inexpensive not on a $1000.00 locomotive. The more he does it the more the more he will learn what techniques works for him. Here is three locomotives I did.