Radak #1: It is a Tefilah. A proof is "v'Al Titnehu"; Al is a supplication and request, or a command. The same applies to "Yemaltehu Hashem", "Hashem Yis'adenu" (verses 2, 4).

Radak #2: It is future; so Hashem will do to the sick - He will spare him, sustain him and make him live; the doctors will not do this. Their cure helps only with Hashem's help. "Al" means 'not', like "v'Derech Nesivah Al Maves" (Mishlei 12:28).

Radak citing his father: So the visitor says to the Choleh - Hashem will spare him and guard him on the day of evil.

Radak: He guards him from death.

Malbim: Hashem will guard him from evils due to lack of housing, clothing and food.

Radak: He heals him; his illness does not stretch out.

Radak: He will do good deeds and succeed, and all will say that he is happy.

Malbim: The rich bring their food and needs from afar. He is happy in the land, through Hashem's Hashgachah; he need not roam to the end of the land to get his food.

Radak: It is his enemies' desire, that he die.

Malbim: It is being handed over to his enemies.