Tell him to buy a 4 stroke...better gas mileage and no mixing....LOL.
Honestly, I rode a 72 mile loop with my stocker motor and bolt ons. Wide open trails, 5th and 6th gear pinned.
At the 45 mile mark, I had about a gallon left.
Keep a bottle of two stroke oil on the bike, backpack, etc. Keep enough for two gallons.
Pull up to the pump, pump two gallons in FIRST....then dump the oil in. Sit on the bike and rock it for a solid 10 or 20 seconds to mix it up.
The reason I say put the gas in first...is to make sure it needs 2 gallons or so...adjust the oil from there.
Either that, or buy a big, ugly oil tanker looking gas tank....
I understand exactly what you're trying to do....long trip, not carry a spare/auxiliary gas can....and pull up to the pump to top off...get fuel.
Figure your stock tank holds just under 3 gallons.....you need to figure out how many ounces of oil (or liters) per gallon, and bring with you the appropriate amount.