Scott,
The hitting-wall problem your cousin has sounds like a common effect of the ridiculous way that stock wheelchairs are programmed - delays going into a turn, and even worse, delays stopping a turn. You can find a very extensive discussion of this and explanation of how to re-program a chair at: http://www.wheelchairdriver.com/. There is also a lively discussion of all things powered-wheelchair, with more technical information and expertise than you'll find anywhere else, at the forums of that site.
Interfacing proximity sensors with the wheelchair controller will probably not be simple, and with good chair programming may not even be needed. The controller communicates by a variant of the CANbus protocol, but the actual protocol and messages are proprietary and the manufacturers just don't give out that information. The controller manufacturer may have a switch-interface module, or other usable external interface module, available, but I am sure that you will faint when you see the price. You can also physically hack into the joystick pot connections within the controller, BUT that will take careful thinking and engineering to avoid introducing safety problems (as well as noise).
I think that the first step to helping your cousin is to get his chair programmed so that it turns without delay and stops turning without delay. Even that is a non-trivial task as the controller manufacturers will almost never sell the OEM-level programmer needed to anyone but a wheelchair manufacturer, and the dealer-level programmers are locked out of the parameters that you need to adjust. For some controllers, people on the WheelchairDriver forums have found ways to purchase the needed hardware/software or have found workarounds. You might even find someone living near you or your cousin who might be able to help.
When you go to WheelchairDriver, do try to not take offense at the sometimes polemical language used. For us, or for our loved ones, the wheelchair should be a massively enabling and liberating device and there is a lot of resentment of what many see as the denigrating and stick-in-the-mud attitudes of a wheelchair industry that is driven more by what insurance providers say than by good engineering. I loudly applaud your intent to help your cousin, and hope that you will take the time to absorb the collective experience of some very committed and imaginative folks at WheelchairDriver, as well as their willingness to help others.
Ciao,
Lenny Robbins
(Siena, Italia, formerly from Lansing, MI USA)
father of Rachele, tetraplegic and non-verbal because of dystonic cerebral palsy