I'm creating a 4 x 6 meter (~13 x 20 feet) outdoor billboard advertisement that includes an image and vector logo (rasterized) in Photoshop. I was working using a smaller size for design purposes, but now the client has approved a final design and wants to go to print. I need to know what DPI/PPI I should export it as.

Additionally, I was given advice by a friend to make it at 30 DPI and work in centimeters (so 400 x 600 cm) then let the print house blow it up to 4 x 6 meters, is this true?