How to address a doctoral candidate who is ABD (All but Dissertation)? Is "Dr" appropriate?
Address them the same way you would address any other person without a doctorate: e.g. in many English-speaking cultures, &quot;Mr. Smith&quot;, &quot;Ms. Jones&quot;, &quot;Mx. Brown&quot;,  or if you are on friendly terms, by their first name.
If they have successfully defended their dissertation but not actually received the degree, this is a bit of a gray area (see When can you call yourself doctor?) but I think &quot;Dr. Brown&quot; is reasonable.