As always, wireless performance is in the eye of the beholder (and, depends heavily on the enviroment in which the router and clients live, as well as the installation itself.)
1. 5GHz applies to both 802.11a, and to 802.11n (dual-band.) N routers can be 2.4GHz, or 5.0/2.4GHz.
Going dual-band might not give your friend anything now; however, it would give him forward compatibility with, say, a newer laptop that he may buy in the future.
2. Reviews, and first-hand experiences.
All I can say is: research, research, research. And, ask people for their opinions.
FWIW: honestly, I can't remember the last time I had to reboot my DIR-655. And, I've had it for over a year.
3. For one, tell him that 'amazing throughput for backups and streaming' also (generally) equals 'amazing performance and reliability for normal use.'
how long is the house? And, how many walls between the one end (where the equipment is) and the other (where he works?)
Any possibility of relocating the router to a more central location? That may help him out with throughput issues, what with removing some walls from the signal path.