Tell us the sequence - which rounds doubled? First two in the clip, five and six, etc? It is relevant, because some Garands eject the clip prematurely or stoppage on the 7th round. These are known conditions with known fixes.
Shortie Garands are handy little guns, but the conversion to .308 adds a few cautions. These "fixes" are a few I remember from other internet discussions about 30-06 to 308 conversions and normal length to short barrels.
For instance, the gas port on a normal length 308 barrel is supposed to be slightly enlarged for better oomph. The solution is to drill out of the gas port, or get an adjustable gas plug.
Likewise the height of eight 308 cartridges in the clip is ever so slightly taller than eight 30-06s, and some receivers have tighter tolerences which make stripping that first round off the clip a real #####. Without knowing exactly where the Garand Master Gunsmiths relieve the receiver, I'd think that grinding the "lifter" a fraction thinner would do the same trick.
The final area for attention is a spacer block in the receiver to keep 308s from sliding forwards in the clip. Not really a problem for 95 guns of 100, but apparently it can be a problem.
Here is my suggestion to explain your doubling Tanker. You answered your own question without knowing it. The Garand firing pin is freefloating. Unlike other guns, there is no spring to keep it out of contact with the primer when the bolt closes. Every time I unload my Garand I cringe at the sight of tiny dents in the unfired cartridges. If your buddy was firing commercial ammo, I'd bet my Father's Day steak dinner that he had a "slam fire" from the firing pin hitting the newly chambered round and its softer commercial primer. The round fired just as if the firing pin was struck by the hammer. The solution is to use harder primers or only fire military surplus.