Trojan says 6.37 volts @ 100% charge on 6 volt on the batteries.
Trojans are the best according to most and a set will cost you $600 +. Most folks I hear of (including myself) run the Trojan 105's.
Your friend probably bought the cart "as is" on the batteries. The best he could hope for is for the cart place to replace the 3 weakest batteries with some more used batteries, but that is only buying time as the other 3 are "tired".
Here's a battery comparison chart, it might be a little out of date but for the most part it is accurate.
6-Volt Batteries
Trojan batteries specifications:
T-105, 447 Min @25Amps, 115 Min @75Amps, 225Ah @20Amps, 62 Lbs;
T-125, 488 Min @25Amps, 132 Min @75Amps, 240Ah @20Amps, 66 Lbs;
T-145, 530 Min @25Amps, 145 Min @75Amps, 260AH @20Amps, 72 Lbs
Exide:
E3600, 390 Min @25Amps, 110 Min @75Amps to 5.25 volts, 186Ah @20Amps, 62 Lbs;
GC-5, 480 Min @25Amps, 135 Min @75Amps to 5.25 volts, 226Ah @20Amps, 65 Lbs;
GC2-H, 525 Min @25Amps, 155 Min @75Amps to 5.25 volts, 245Ah @20Amps, 68 Lbs;
NAPA 8144 (mfg. by Exide - equivalent to Exide 3600)
390 Min @25Amps, 110 Min @75Amps, 186Ah @20 Amps, 62 Lbs
NAPA 8146 (mfg. by Exide - equivalent to Exide GC-5)
480 Min @25Amps, 135 Min @75Amps, 226Ah @20 Amps, 65 Lbs
StowAway STGC2 (Mfg. by Exide for Sams Clubs - equivalent to Exide E3600)
390Min @25Amps, 110 Min @75Amps, 186Ah @20Amps, 62 Lbs
US Battery:
US-1800, 392 Min @25Amps, 107 Min @75Amps, 208Ah @20Amps, 56 Lbs;
US-2000, 445 Min @25Amps, 114 Min @75Amps, 216Ah @20Amps, 59 Lbs;
US-2200, 474 Min @25Amps, 122 Min @75Amps, 232Ah @20Amps, 63 Lbs;
US-125, 517 Min @25Amps, 140 Min @75Amps, 242Ah @20Amps, 67 Lbs;
US-145, 562 Min @25Amps, 154 Min @75Amps, 251Ah @20Amps, 70 Lbs;
Deka Batteries:
GC10, (? Min @25Amps), 100 Min @75Amps, 190Ah @20Amps, 59 Lbs
GC15, (? Min @25Amps), 115 Min @75Amps, 215Ah @20Amps, 63 Lbs
GC25, 488 Min @25Amps, 132 Min @75Amps, 235Ah @20Amps, 67 Lbs