cummins isx cm871 not starting please help

Hell everybody,
Sorry if the story is confusing but it's what I'm getting from my drivers over in Phoenix in an 08 prostar , they stopped at the gate of a UPS warehouse and after they were ready to go in the truck wouldn't start back up, driver said truck was cranking very slowly and then it went to just clicking, they checked the connections at the batteries and tried giving the starter a couple whacks, they got another slow crank then back to clicks driver also said there was a burning smell, the voltmeter was reading thirteen, I coulda went out there myself as im qualified to work on it but I'm in Fontana, ca, I called a mechanic to go out there and he went ahead and changed the starter, but it was still having the same problem, he checked the fuses and relays and the cables and checked the batteries but it all came back good, I have no clue and can't see the truck but I'm going on my way right now to check it out before I have to get it towed, any input would help, I hope it's not hydrolocked or seized and hope it's something electrical, anything will help thanks

 

Hopefully that's it, I'm almost there, thank you

 

sorry for the late response, had a baby born, yeah it ended up being very stupid, the ac compressor clutch was seized so the belt couldnt spin around it and wasnt letting the engine crank, what did urs end up being