Coauthoring with a reviewer possible? I recently submitted an article to a math journal and got a response
from a reviewer, with an attachment. In it, I have found some good 
advice that could improve my article tremendously.

I am a bit suspicious of its accuracy since it could resolve
what I have been working with for some months in a few 
sentences. 

But anyway, it was enough for me to ask the reviewer for
coauthoring with me. 

My question would be, does it often happen that
an author asks a referee for coauthoring during
the review process? Or could it even be possible?

I was given a month to revise my article, but
the chance that I could revise successfully
by myself seems to be fat.

I would be looking forward to any advice.

Thanks in advance.
Yes, it is possible.

If you want to invite the reviewer to be a coauthor, send a request through the editor.  If the editor agrees, they will forward your invitation to the reviewer.  If the reviewer accepts, they will reply to the editor, who will introduce the reviewer to you, and off you go.