Tell you friend to try some little can mufflers in the tailpipes. Magnaflow or similar.

I'm going to guess that he is like me and will hate the drone that most any mufflers will cause. So he will need the can muffs as resonators later anyway. They might cut the sound back to an acceptable level even with the Flowmasters. The sound will change some too as the last mufflers in the system will influence the exhaust sound a bunch.