Does he have normal kidney functions?

The most common deficiency in the elderly is B12.
In fact the NIH recommends that all persons over 50 take a B12 supplement.

Many drugs disrupt nutrients in the body.
This site explains it:
http://www.enotalone.com/article/4624.html

I have a reference book that is more technical than the above website and looked up your father's drugs.

Digoxin affects:
calcium
phosphorus
magnesium
thiamine B1

When B1 gets low, mental functions decline. So I would focus on that first.

Glyburide depletes CoQ-10.

Low CoQ-10 impacts the heart and other muscles, and reduces energy from the mitochondria in cells. (these are the energy factories). CoQ-10 is normally made in the liver but that may fail with age, or certain drugs like statins for cholesterol.

B12 and Thiamine are what I'd start with.
Get a good activated form of B12, called methylcobalamin and take one daily on an empty stomach. You can have his B12 measured with a blood test, if his doctor is cooperative.
This form of B12 is often not in stores so we all buy it online here:
this page shows the various choices:
http://www.iherb.com/Search.aspx?kw=methylcobalamin
most of us use Jarrow, but you can choose anyone you want.
Start at 1mg daily.

The thiamine B1 is available in most stores for a very small price. I'd start him on 100mg a day, and if he perks up, increase to 300mg a day in divided doses. Thiamine gives signs when it is excreted. The urine is more yellow and has a distinctive odor--yeasty. If he excretes it quickly within 4 hrs that is a good sign of adequate kidney functions.

Both of these are easy to tolerate and are often used in the elderly.

Diabetics lose magnesium in the urine, so he will need some of this as well. But you need adequate kidney functions present. I have a magnesium thread:
http://neurotalk.psychcentral.com/thread1138.html
Until you know his kidney status, I'd use food sources for this mineral.

The use of Lanoxin implies to me a degree of heart failure.

The use of warfarin limits him tremendously in the herbal area.
Just about everything interacts with warfarin! (as you know)

This site is very good about supplements and interactions with drugs:
Here is thiamine for example:
http://www.umm.edu/altmed/articles/v...-b1-000333.htm
notice the interaction with digoxin.
It is ironic and typical of many drugs...that are used to treat a condition..end up causing it. Depletion of thiamine actually makes heart failure worse.

This is B12:
http://www.umm.edu/altmed/articles/v...b12-000332.htm

I don't see Warfarin on either monograph.

I worked in nursing home care for over 8 yrs...and just about every patient (we serviced 35,000 beds) was on B12.

The other issue with your father is a possible thyroid condition. The elderly become hypo commonly. But that requires blood work to evaluate.

Please read the links I've given. And if you have any questions don't hesitate to ask.