She says: “Many people are unaware that lemon juice is actually alkalizing for the blood because while it is indeed comprised of citric acid, it leaves an alkaline ash.” Is this true, and a problem?

We see this as a non-issue entirely. Everything taken in orally is acted on chemically by the digestive system, and that will alter the natural pH of the substance to some degree, and this is a normal part of the dietary process, to condition food to be in the best form for absorption via the digestive system and subsequent utilization by the tissues.