Is this postural instability caused by by the Parkinson's disease or is it related to syncope or orthostatic hypotension as a side effect of medications taken either for PD, a cardiac contition or another issue? Postural instability can also be a byproduct of depression for which PD does not lack.
There is little help through medication. Although I do not trust a lightweight walker, it may be of some value. A cane with the legs at the bottom might do the job - two might work even better. There is value to having handrails installed in hallways and even bathrooms just as they are on stairways.
One important aid is exercise. Gait exercise can be crucial. I suggest asking for a referral to a therapy center so that gait training can be initiated. Gait training would include lenghtening of stride, learning to open at the hips for a wider stance. The smaller the space between the legs, the less option to maintain balance. Lifting the legs almost in a march rather than shuffling and sticking to the floor.
Other exercise is also important. Range of motion, flexibility stretching and yes, even strength training and breathing. Yoga exercises can help. Nautilus is important. There are small pieces of equipment which can be purchased for home use. Don't neglect hand exercises. Being able to grip is crucial.
I am wondering if Dance Therapy might not be helpful. Although its effects will not be physically apparent after the music stops, there might be a psychological-confidence effect which impacts postural instability due to PD depression. And it is F U N! You can both take part. Even in your living room. Get those endorphins going.
Another possibility is to locate a therapy center which has Forced Exercise equipment and aqua therapy. Water exercises can be of benefit to a PD patient because of the bouyancy effects of exercising-marching in the water. And Forced Exercise - not on a treadmill in this case unless there are trained spotters - but on the motorized exercise bicycles which compensate for the patient's inability to maintain the necessary rpms on his own. For some people these bikes are an affordable option for home use - for others, they are not. But the bike must be able to maintain between 80-90 rpm.
Naturally there are aids such as shower seats (with backs or transfer seats), grab bars in the tub/shower area (no suction cups please), hand-held showerheads with longer hoses and on-off switches to prevent unnecessary standing in the shower. Soap on a rope.
No PD home should be without bath/shower aids even if they are not needed immediately. Knowing how to use them and using them regularly helps to train the body so that when the equipment is needed, there is kinetic memory to reinforce the use patterns. (Yes, I am a believer in the possibilities of kinetic memory to resist the tightening muscles and I have seen it work with my husband in entering/exiting the shower after the side effects of dehydration - another story - had some temporary but dire cognitive effects)
I think that not nearly enough is available to address postural instability in PD. PI is discussed as a later symptom because it is apparent then. To many it is an invisible side effect as a very early symptom. Everyone talks about falls, the risks, prevention and the role they play but we haven't found many solutions to address the problem by fixing the chemical imbalances which cause the problems.
At this point prevention and exercise seem to be the best methods. One exercise that should be included is practicing turns on the floor (when assistants are present to help the patient rise) By repeatedly rehearsing turning to a position where one can pull up to a kneeling position and then make use of a solidly built walker with brakes, this proud man could learn to help himself and his wife. (Another argument for the other forms of exercise accompanied by some strength training).
One last thing, this proud man might take better to the exercise suggestions if he understood that he was doing it not just to maintain his body but also to benefit his wife's peace of mind, security and physical safety from injury. And wouldn't it be wonderful if it helped.