That's a little unusual that it would be "set" to single or multi-player.
Normally, in arcade play, you would add coins then press "Start" once for each player.
Or if set to free play, just press "Start" once for each player.
Maybe he has an odd way of setting to free play.

If anything, the stepup units could be frozen. "coin unit" tells the machine how many players are per a game...
if the unit does not advance then it will stay on the first player.
In the back box is the player control unit.. it does resets, ball in play, and moves from player to player.

Probably that infamous old grease.

Target Alpha is a good game and a terrific first machine. Make sure the coin unit is even in there. Stranger things go missing over time in older machines. There is no reason for the machine to be set for 1 player.

Weird . Ask him how he did it. If legit and nothing’s seriously busted, EMs are all about tweaking and adjusting. You should be able to reverse most anything. If One Player setup means he ripped the guts and score wheels out of the other three players then I’d pass (unless it’s a steal and you can afford to replace any parts he doesn’t have)