At the speeds you are talking, there shouldn't be much problem. You will have to stop every 100 miles for him to get gas. I ride with the Patriot Guard Riders, and I have been on 400 mile round trips with them in one day. We will normally stay around 60 - 65, some times 70 for short periods on the highways, and I have not had a problem. On one ride, we had a Highway Patrol escort for about 70 miles at around 75-80, and my 06 S40 did OK. At around 60-65 MPH, the S40 will be fine. I am 5'10" 220 in leathers.
As far as gas milage goes. I get around 58 to 60 MPG at around 60, and about 54 to 56 MPG at around 65 to 70. The best I have got was 63 MPG, which was on a little 100 mile ride I went on on country roads doing around 45 to 55.
After around 100 miles he will be wanting to get off and give the backside a rest for a few minutes, so that works out good because of gas stops. After 300 to 400 miles in a day on a bike weighing only around 360 wet, he will be ready to call it a day. 400 to 450 in a day is enough for my on the S40.
One more thing I will tell you, is if you have to time before you go on this trip, have him get a Superbrace. The difference it makes is tremendous. It took me approximately 20 minutes to install it, and that is including cutting off the factory plastic fork guards with a razor knife. I bought my Superbrace on line from Superbrace.com for $182.00 including polished stainless fork guards and shipping/handling.
8)Have a safe trip, and I hope this info has been some help.