Hezekiah, a king of Israel who devoted Himself to God, also became very sick and was on his death bed. He survived by the grace of God, though he did become very sick. Psalm 91 doesn’t guarantee that we as humans will not experience sickness in our lifetime. Not relating you to him by any means by saying this, but Satan tried to tempt Jesus Christ by taking Psalm 91 out of context a few verses down in verses 11-12 in the temptation in the wilderness, but Christ put him in his place and reminded Him that no one shall ever put the Lord to the test. What I’m trying to say is that I think context is very important in interpreting the passage you’ve shared, and also, by praying for your friend, you are interceding on his behalf as a brother in Christ for healing from his sickness. Just because he has covid doesn’t guarantee that God’s presence is absent from him nor does it make Scripture void