A friend with an '07 RT1200 is, again, stuck with a failed gas tank level sender. It was replaced under warranty. The current sender lasted longer than the others before failing.

I wonder if it's the sender/sensor that's failing or wiring wearing through.

Does anyone have an insight into what's happening here?

I am pretty sure that model has the notorious BMW Fuel Strip mechanism in the tank rather than a traditional float.   BMW extended the warranty on these to 12 years because they continually fail.  They never provided a fix, just a warranty replacement with the part that is known to fail.   It is the microscopic resistive wires in the flexible strip which fail, nothing mechanical and nothing in the regular wiring harness.   

My 2007 GS Adv has been through five (six if you count the one that was DOA when the dealer installed it).   The last failed this Spring just after the warranty expired.    I now just reset the trip odometer and the fuel mileage every time I fill up.    The fuel strip is almost $200, not counting the charge for dealer installation and computer calibration.   So, probably $400+ for a replacement with a full expectation that it will fail again in short order.

A guy in the Netherlands has supposedly worked up a way to use a later model float and recalibrate the computer to match, however you need to buy those parts, plus an Android phone, the app to fool the computer, and adapter cables from the phone to the bike.   I just built a resistive circuit to trick the Low Fuel warning light circuit and live with the trip odo.

My friend's been cut loose and now has to pay for buying another "soon to fail" strip. As you say, time to resort to tracking miles and being paranoid about the amount of gas on board. Or... "never pass gas".

What's involved in at least bypassing the strip to turn off the warning?

This has been a known problem for many years so people have come up with a couple of ways to fool the computer into thinking the tank is full.   One is cutting the wiring harness going to the fuel strip and splicing in a couple of resistors.   The other is to disconnect the cable connector from the fuel strip on top of the fuel tank and plug in a homemade circuit which the computer sees as both the strip heater and the fuel strip.   I chose the latter method simply to keep the stock harness intact.   Either method only requires simple soldering and heat shrink skills.    I'll post a quick schematic of the resistors and the connections for that second method.   

The computer basically sees R1 as the strip heater and R2 as the fuel strip resistance with a full tank.   It is likely possible to use a different resistance for R1, however that is the range others have found to be successful so that is what I used.   All of this is detailed in long threads on several different BMW boards in Europe (where they did not get the extended warranty), as well as on AdvRider.   

I probably still have extra resistors of each type somewhere in the shop.   Assuming I can find them, I'll be glad to send you or your friend one or two of each if you PM me an address.   I am not back home until the weekend so it will take a few days.  Otherwise, they are available at Frye's, Amazon, etc.

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Thanks for the diagram - no worries about the resistors. How did you tie the resistors to the existing tank connector?

Thanks! IMHO bodge may also apply to the sensor assembly. IMHO, that is.

Thanks for the link.

It's disappointing that BMW didn't have the sense to replace the strip, known to fail, with something more reliable. Sigh...

It's underwhelming to have to re-engineer part of the bike to get around a "seemed like a good idea at the time" design. At least it's not my bike. 

Being unkindly blunt about it, the BMW of fame and glory (two or four wheels) is a) probably gone, and b) probably never existed.
"This BMW machine is fantastic! Lives forever!"
"Who told you that?"
"BMW!"