Well, a lot of Ballasts are listed to drive various lamp powers - so they range from a low lamp wattage, a medium wattage, and the highest one is the maximum.
For instance, a typical two lamp Electronic Ballast can drive 2 F17 T8, or 2 F25 T8, or 2 F32 T8 lamps.
If a Ballast is listed and spec'ed this way, it can be used on various lamp powers.
Now, as to the situation your Friend experienced, capping off leads to one lamp's circuit of a Rapid Start Hot Cathode Magnetic Ballast[or even a Magnetic Hybrid Ballast] is almost impossible to drive a single lamp. From the sounds of things, since the Thermal Protection is tripping, this Ballast is connected incorrectly. The most likely suspect would be a short on the "Secondary", which is what the Yellow Wires are derived from. These Yellow wires perform a dual function:
1: They keep the Cathodes on that side of the lamp hot during lamp operation,
2: They complete the Series connection between the lamps and Ballast.
If you have ever removed one lamp from a two lamp T12 setup while the lamps are on, you know that when one lamp is removed, the other one goes out. That's due to the Series Connection between the Lamps and the Ballast's "Primarys" and "Heater Windings"
Review this Schematic for more details: F40T12 2 Lamp Ballast Schematic
The ratings you listed for Lamp types hint towards this situation being for a 6 foot to 8 foot Lamp.
Is this for an HO, or VHO Lamp, or just a Standard 460 ma. Lamp?
Also, is the Ballast in question supposed to drive HO / VHO type lamps, or just Standard 460 ma.?
If the Lamp is a Standard, Single Pin, 460 ma. Lamp and the Wholesaler gave your Friend a Standard type Ballast, this setup will work without tripping the Thermal Device. It would appear to the Ballast as if the 2nd Lamp was no good.
Now if the Lamp is an HO or VHO, trying to drive it from a Standard type slimline [sequence start] Ballast would be odd, since the Lamp will need Hot Cathodes, or an extremely high Starting Voltage. Not that it can't be done, but it would result in odd Lamp behavior.
On the flip side, if trying to drive a Standard Single-Pin Lamp from an HO / VHO Ballast, it's unlikely that the lamp would even start, plus the chances of short circuiting Ballast leads would be very high!
Also, you would have to cap more than just one lead!
Like Scott [electure] said, 1 Lamp Mag Ballasts are still available. In California, we cannot install Magnetic Ballasts with the Rapid Start function [Hot Cathode] in new Commercial / Industrial projects. Nothing totally prohibits the use, but the restraints of Title 24 part 6 [energy conservation calcs] makes it much easier to utilize the Electronic High Frequency Ballasts driving T8 Lamps [or even T4 Lamps]. As long as the Ballast and Lamp Efficiency is high, plus the items are listed by a CBM as Efficient, they can be used.
This subject is kind of complex and in-depth, so let me know if there are further questions regarding Title 24.
Have your Friend take that Ballast back to the Supplier and exchange it for an equally usable 1 lamp Ballast, which matches the type of lamp being driven. It could be a 1 lamp magnetic, or a 1-2 lamp Electronic.
If the supplier is still reluctant, get the money back on the Ballast, then go to the "Big Orange Box" [AKA Home Depot] - or other DIY'ers outlet. They might have a variety of Mag Ballasts on the shelves.
Hardware Stores most likely would too.
P.S. While your Friend is at the supplier, have your friend ask "Just why are'nt there any more Mag Ballasts Available" and "Where Did You Get This Information From".
The supplier has got things kind of backwards. Lamp Manufacturers are not mass producing T12 Fluorescent Lamps, due to the 1996 Energy Commission and EPA Mandates. Existing Lamps can be sold and used in existing fixtures, but not for new stuff.
Ballast Manufacturers can still make and distribute Mag Ballasts, which are targeted towards home users and also "Legacy Fixture" maintenance.
The trends have shifted towards a wide acceptance of T8 Lamps being used for the most common Construction Projects [AKA Commercial], so they have a higher ratio of Electronic High Frequency Ballasts produced and in-stock.
Last note, if Magnetic Core Ballasts are no longer being produced, why is it that I still recieve HID fixtures with Magnetic Cores?
When we moved Advance Transformer's West Coast Distribution Center from Fullerton to Chino in April of this year, nearly all the HID Ballast kits were Magnetic Core types. Only a small percentage of them were Electronic - and those were Hybrids!
Have your Friend throw that one at the Supplier!