Before you read any further, make sure to look at the title of this entry. The word could be invalid as in bedridden or it could be invalid as in unfounded, worthless, or null. I shudder whenever I hear the term used in the former denotation of a person who is weak or ill because it so easily could mean the other as well. Is the term on some level a denial of personhood? While I suspect the answer is yes, I have no logical argument to prove it.
Now how about another favorite of mine: wheelchair bound. Users of wheelchairs are typically unable to get themselves from point a to point b solely under their own power, so they plop themselves into this device and suddenly can move about with freedom. Does that experience fit with the word bound? Replace it with the grammatically appropriate version of the word use, as in wheelchair user or person who uses a wheelchair and suddenly something more reflective of reality.
There is a place between saying anything and scrutinizing each word endlessly. I wish people would strive for that place. To most achieving it feels daunting, so they give up and stop referring to things forthrightly. In my opinion, it is far more doable than you think.
Start with the premise that the person you are referring to is simply different from you. This means you acknowledge human variation and the atypical way the person interacts with the world but you do not judge or rank it. Next determine the concept you wish to articulate and attempt to find an equivalent in your life. For example, you move from point a to point b by use of your feet, a car, bike or bus. How would you describe that? Hoofing it, riding in a car, riding a bike, or bus rider would work. None of those terms imply you are glued to your method of locomotion so why would you describe a wheelchair user in those terms?
The term bound is one that probably can be traced back to somebody thinking about how they, a TAB, would feel if they needed to use a wheelchair and could only come up with the feeling of trapped. Don't try to put yourself in the shoes of a disabled person when it comes to judging their life or how they feel about it. Simply put, it cannot be done in a neutral way free of societal beliefs about ability and physical difference. As a TAB you are also unaware of the techniques we are taught to accomplish tasks, emotional changes our disability has facilitated, and what windows might have opened when doors closed. In other words, you lack basic information necessary to occupy our footwear.
It is far better to seek an equivalent concept in your able-bodied life and find a way to describe it that would work for you. Here's a great example. From time to time, friends of mine find themselves in the position of wanting to know if a person they've met is acquainted with me. Usually, if this happened with a non-disabled person, a description of the individual would be given. To me that makes it completely reasonable and efficient to say, "Do you know Jen? She's blind, has curly hair, and her face is pretty unusual." It's the equivalent of saying, "He's tall, really skinny, and has long hair."
Back to invalid. Usually that word refers to a person who cannot get out of bed or leave the house. If you treat that as a fact that has no inherent implications about the person's quality of life, then I imagine you can come up with a better term than one which means null with a change in emphasis. It is probably going to take a few more words, but I think it might be worth the effort.