I think he's probably looking at PA speakers for use with his guitar, that's what I'm guessing at least, going by what was said about 600w. I'm also guessing that he's looking at those because he's thinking more watts is better, and you can't get that kind of power without going for PA system.
More watts definitely isn't better; unless it's for use outdoors or in a huge room or hall, such wattage power is really not needed. For use in an average sized room, much better to buy studio monitors. PA speakers are fine for volume in large or open areas, but the quality will be sorely lacking compared to good studio monitors.
Anyway, enough assuming and speculating.
I wouldn't get something like a Dragonfly, because that's output only. Great for music use, but not a good choice in this instance, as he will still need to have his guitar connected to the motherboard audio.
An audio interface would be better, something like a Focusrite iTrack Solo; just leave the other speakers connected to the motherboard for gaming duty.