I think he should stick with 6GB of memory if possible. He should just get slightly slower memory, the i7 creates enough memory bandwidth to overshadow the slower speed and or latencies.
He'll still save a little cash with a set of these.
Newegg.com - SUPER TALENT 6GB (3 x 2GB) 240-Pin DDR3 SDRAM DDR3 1333 (PC3 10666) Triple Channel Kit Desktop Memory - Desktop Memory
Indeed. Depending on what he is doing for video editing, he may need all the RAM he can get. Speed shouldn't matter as much, but capacity will probably make a difference.
Though, it would be good to know if he plans on using a 32bit or 64bit O/S. 6GB of RAM with a 32bit OS is a waste of money.