2005 Audi S4 (4.2l) O-ring changed on fuel pump -- now won't crank / turn over -- Starter? Wiring? Relay? Fuse?

[jimmy154]

What could it be?
2005 Audi S4 convertible 4.2 liter V-8

My friend changed the O-ring that leaked on fuel pump.
Started and ran then moved out of the garage, a few feet to a parking spot.
Then the next day. The starter does not turn over.
Everything else seems normal.

I'm not at the car right now.
What could it be?
Automatic immobilization relay? Does it have one? (had this issue on a e34 BMW)
Starter relay?
Starter fuse?

I will measure the starter when I get to the car; it's my mother's car.
Just wondering what it could be before I get there, so I know what tools to bring and so I'm not lost when I get there.

 

[NHN]

Assume battery is ok as leaving ignition on can flatten quickly with xenon's, could be that.

 

[jimmy154]

Short jumper cable on the starter was corroded, for any one having this same issue.