You feel well again!

[Silverobama]

A few days ago a friend of mine caught a cold and felt very sick and said things in a very low voice. Last night I saw him at a local English club again and he recovered! I said "You feel well again!"

Is it natural? If not, what can I say？

 

[emsr2d2]

A few days ago a friend of mine caught a cold, and felt very sick and said things in had a very low hoarse voice. Last night I saw him at a local English club again and he had recovered! I said "You feel well again!"

See above. Note that "feel sick" in BrE means to feel nauseated (as if you're going to vomit). If you mean "unwell", use "ill" in BrE and "sick" in AmE.
You needed the past perfect "had recovered". Using the past simple suggested that the mere act of you seeing him at the English club caused him to suddenly recover. The past perfect makes it clear that the recovery happened before you saw him.

Is it natural? If not, what can I say？

Not really. We don't usually tell people how they feel. I'd say one of the following:
Are you feeling better?
You look better [than the last time I saw you]!
Have you recovered?