Greetings esam!
Your father's body is attempting to cleanse itself of mucous built-up by many years of incorrect eating. As unpleasant as this must be for him, it is imperative that he trust his body and aid it as much he can in its quest to purify itself. This may mean weeks, or even months of discomfort; however, and as harsh as it may be to say, he brought this upon himself and must suffer the consequences if he wishes to restore balance.
Unfortunately, when people eliminate certain foodstuffs, they can sometimes expect to see and feel dramatic change within only a few days. And if this change does not arise, they proclaim failure and either return to their old ways or seek an alternate remedy.
The fact is, the phlegm/mucous is unnatural, and hence was caused by the eating of unnatural foods. If your father eats correctly (i.e. fruits, herbs, nuts and seeds, in their correct combinations and at the correct hour of the day) then the mucous will dislodge, dissolve and be excreted; via the bowel, the kidneys, possibly even the skin and almost certainly up and out of the mouth!
If you have not already done so, please read as much Arnold Ehret material as you can, as well as Dr. John R. Christopher's text on the Mucousless Diet. Both writers arrive at the same conclusion, although Ehret may be seen to be a little more "extreme" in some cases, and indeed, this is what differentiates him from the rest of the pack.
To hasten the break-up of mucous, a protracted fast could be undertaken, although depending on the overall condition of your father's body it may be wise to fast for three days only, prior to commencing the "perfect" diet. All you could ever need to know to defeat ANY health condition can be found in the works of Ehret and Christopher, and a combination of correct eating and herbal formulae is the answer to all health concerns.
If you require any additional information please do not hesitate to ask.