starter wire

[1990SSchevy65]

Hello
I own a 1965 chevrolet truck. The PO swapped out the I6 and 3 on the tree for a 350 and an auto. I think the PO wired the truck wrong because I can start up the truck all the time I want. The only problem is I let a friend borrow the truck twice now and both times when I want the truck back she will not start at all. The first time I had to replace the starter and the alt., this is the second time and when I went over there to try and start her up you could hear the starter trying but wouldn't start. Now, I did find out that the friend would shut the truck off in the accs. position even though I told him that the key needs to be straight up and down then pull the key out, but he did not do that. My question is (sorry for the long post for this) is the alt. bad again or could the wiring job the PO did have something to do with this? Not the positive battery cable wire the small wire that I believe is purple? Sorry being colorblind doesn't help me much sometimes.
Thanks in advance.
Scott