See post #27, #68, and #72.Ahh so the solenoid is integral to this starter? hmm... in that case, depending on the design, you can either hardware the solenoid internally, or just bypass it externally if the wires are external. Whatever looks prettier. Then install the new solenoid upstream. Many larger motors use stand-alone solenoids, so they are easy to find. Now getting your cheapskate customer to pay for it, you're on your own with that one.
No solenoid is fail proof, that's why they sell replacements. But if you get one with much greater capacity than what is required, there's probably less chance of the failure mode being stuck contacts:
http://www.amazon.com/s/ref=nb_sb_noss?url=search-alias=aps&field-keywords=starter+solenoid
But now I understand your point, that you customer might just want a panic switch of some sort so he can disconnect power quickly IF the problem happens. Maybe something like this would be a nice solution (if you can shake his wallet free):
http://www.amazon.com/Fastronix-Cur...7&sr=8-1&keywords=high+current+battery+switch