I suspect the problem with the centers of the large canes you see that appears to be rot could actually be insect damage, possibly from cane borers.

We cut the cane back bit by bit to below the damage -- fresh white centers indicate live wood. Some rose friends take the time to dab the fresh cut with white glue to protect it from the borers.

As far as rot at the base of the plant, this could be canker or another problem.

Fungus spores exist in the rose garden no matter what you do, but they only multiply and cause damage under conducive conditions, including lack of sunlight & poor air circulation.

Warm, dry days and cool damp nights encourage the growth of powdery mildew. Cold, wet weather encourages the growth of rust fungus.

Some varieties of roses are more susceptible to rust and powdery mildew fungus, while other varieties are resistant.

This does not depend on the age of the bush, although the stress of neglect can be a partial factor. A disease-prone variety will not spread fungus to a disease-resistant variety to any great degree, even if planted right next to it.

If your neighbor removes rose bushes that show serious disease problems in your climate, she will have an easier time. Your local rose society could be a source of information on varieties that resist disease in your climate.

If the plant has lumps of canker at the base or only has one or two live canes remaining, I would think she's better off without it.

I agree that more mulch is needed -- three inches of composted mulch will help a lot. In this area, composted redwood bark is readily available. I add two cups of alfalfa meal under each large bush as a growth stimulant. In my area, 1/4 cup of Epsom salt under each bush makes a nice spring tonic, remediating any lack of magnesium in the soil.

On the west coast, late December through January is a good time to prune (in your neighbor's case, re-prune properly), scrape loose bark off the base & bud union, clean up any dead leaves off the beds and dormant spray.

After my winter pruning I use Cooke's Kop-R-Spray to deter fungus and Volk horticultural oil to smother insect/mite eggs.

During the growing season, I also spray the foliage with a dilute solution of liquid seaweed (kelp) emulsion about once a month, which boosts growth and improves the foliage.

As far as what you should charge to fix up and maintain her garden, I have no idea, but she is smart to ask for your help!