Hello again!
I have written a patch for the bug #47234: tabbing in file browser works only in the current working directory. Here is the link -
http://savannah.gnu.org/bugs/?47234.
For now, I have set-up a global variable to pass path from do_browser() to cwd_tab_completion(). I preferred not to pass it as an argument as there is another function - do_prompt() - sitting between them. I don't know if altering it, too, is favourable as it is also used elsewhere.
Patch is over nano-2.5.3 and I did test it.