invalid there is a noun.

invalid (n) - Someone who is incapacitated by a chronic illness or injury.

Having this said, his wife seems to be very sick, in a crucial condition that might have made her incapacitated.

Now since there's discussion about the degree of being incapacitated (which makes you ultimately invalid), I'm adding a bit to improve this answer.

Here is another reference from OLD:

invalid (n) - a person who needs other people to take care of them, because of illness that they have had for a long time.

Now, if you look at both the definitions, you see that the term invalid ranges from someone being assisted by others to walk, eat or do routine activity to someone who is permanently bedridden (as in the last stage of cancers). Contrary to what Doc and FumbleFingers, it is not always necessary that invalid person is so so so sick that he/she is on the deathbed. And, I'm a doctor and have come across many such patients with chronic illness (in fact, have worked in hospitals that only take such cases).

The OLD further explains it in its example:

She had been a delicate child and her parents had treated her as an invalid

Furthermore, delicate here means:

delicate (n) - (of a person) not strong and easily becoming sick

That's where the WordWeb definition fits in. Invalid is someone who is incapacitated - not able to perform their tasks because of illness that has brought weakness. Here, the child does not necessary to have Ryley's tube or Folly's catheter as Doc mentions.

On the other hand, invalid does not always mean that the person is just incapable to do things and is not so critical. That's why I said, the term applies to incapacitation and this varies from degree to degree depending on the illness that person has.

Check this here:

If you see Saturnino Soncko (a person working in the silver mines of Cerro Rico), he's certainly invalid but I can still argue and deny calling him invalid as at least he is not that incapacitated! In that picture at least he is sitting without any assistance whereas invalid requires support even for this, don't they? They certainly do I see the woman every day. She is an invalid and cannot move anything other than her eyes.

Again, invalid is certainly a serious condition but it varies in degrees or severity depending upon the type of illness. I'm not sure to apply partially invalid or completely invalid for that though it might make better sense.

In US usage, all three terms have been applied historically to individuals who cope with illnesses or conditions, temporary or long-standing, that affect the manner in which they are often used to distinguish them from the bulk of the population in good health.

Historically, these terms have each been used as a label for an individual (or groups) who are perceived to have some condition, either physical, mental or emotional, that affect their activities of daily life. These terms are usually limited to conditions that the speaker considers to be a material impairment of ability.

This Google ngram shows the pattern of usage of the terms invalid, handicapped, disabled and disability.

Invalid is probably the oldest term for someone with physical conditions that are considered seriously limiting. It seems to be used primarily as a noun. in also has a heteronym (with emphasis on the second syllable) that is unrelated to this usage.

Handicapped appears to have come into usage around the late 19th century and has been used both as a noun and adjective. It also has many uses other than to describe individuals with certain conditions. The term had a significant increase in usage in the 1970s as legislation and other programs began to be implemented to improve access to work, transportation, education and other areas for individuals who needed some accommodations to utilize those services.

Disabled has a long usage as an adjective, but it has other meaning unrelated to human condition, such as disabling a machine or a function. It has been used as both an adjective to describe people with certain conditions as well as a noun to label those people.

Disability is a noun which also has a long history and meanings other than the condistions discussed above. It has recently become a more widely used term, often used in phrases such as a person with a disability rather than a disabled person. This discussion reviews the term and the range of conditions that may be covered.

The term invalid is not currently very widely used in the US (but its heteronym is). The terms handicapped and disabled as nouns have fallen into disfavor as descriptor of individuals who have certain conditions, largely because they tend to define the individual by the perceived limitation. Phrases such as people with disabilities, people with handicapping conditions, people with special needs and similar phrases are generally considered more acceptable.

The terms lame, crippled and cripple would generally be thought of as pejorative in most usage when applied to people in the US (but probably not to animals). Their level of usage is fairly low as shown here.

Your friend might be thought of as a short-term invalid, but the usage would not be very common. The use of lame might be used for a temporary injury. The term crippled would probably be considered impolite, but it is occasionally used especially for some conditions, especially as a verb

He has been progressively crippled by his advanced arthritis.

Using it as a noun or an adjective to describe the person who probably be frowned upon.