Nick above is correct. Nearly all Garands have been re-arsenaled at some point, so there is virtually no chance it is "all original". Odds are, they just mean that it is still in the condition that they got it. If you can get pictures, a "build sheet" or a detailed description, that would go a long way towards determining value. However, in general, serviceable Springfield Garands are still available through CMP - albeit with a current delivery wait time of 2-3 months. Field grades go for $650 and Service grades go for $750 from CMP, so that is the starting point for pricing. If you don't want to join the Garand Collectors Association online ($25), fill out 4 forms and wait several months for one from CMP, then figure out how much you think that is worth and add it to the price. The value can also go up on secondary, if they have any rare or desirable parts - assuming you and/or he are sophisticated enough to identify and price these parts appropriately. Without getting into specific rare parts, generally, if the rifle has original GI lockbar sights, you can probably add about $50 to the value. If it has an original single slot gas plug, then you can probably add another $25. If the op-rod is uncut, then maybe add another $25. If the stock has the original cartouches and inspector stamps, then you can add another $50. If it has a 3 or 4 digit serial number, that also generally increases the value somewhat, if that means anything to you.
For negatives, if the barrel is import marked (Blue Sky), then I'd likely pass unless it was cheap and/or immaculate. Blue Sky stamped some barrels so hard, it warped the bore and many other barrels (and rifles) were just in poor shape when imported. Also, if the serial number is 7,000,000 or above, it is a commercial (not GI) rifle and those have cast receivers and are not terribly desirable either.
As for condition, I'd look it over for obvious condition - preferably taken apart to inspect the individual parts for cracks/wear/damage. I'd look for rust/pitting on the receiver. I'd ask if it was originally a CMP rifle, and if so, if they have the certificate. I'd also ask if they know the throat and muzzle erosion numbers. I'd carry a borelight to inspect the chamber and rifling, and also carry 30-06 round to do the redneck muzzle test. I'd remove the trigger guard to see how tight the lock-up is. If it is loose, or too easy to remove, then that will negatively affect accuracy and is generally indicative of a lower grade rifle. I'd also remove the gas plug, and check to see if the gas tube is loose at all or tight. I'd do the "tilt test" to make sure the op-rod was not bent and would carry a caliper to check the tab and piston dimensions to ensure it was not worn down. Also, look really closely at the receiver to see if there are any signs that the receiver was a "re-weld". I'd also check the bottom of the barrel/receiver ring for evidence of a tack-weld indicating that the rifle was assembled from a recovered drill rifle. In the end, if the rifle is okay, then condition and market will drive the price.
