I received a lot of help from an active Stack Overflow user in my undergrad summer research project, which was based on statistical physics. Now, I'm writing a paper on it under my professor's supervision. The professor knows about the help I received and asked me to mention it in the "acknowledgements" section (and the Stack Overflow user is OK with me mentioning them in the acknowledgements). But, I feel that the amount of programming help I received from the user deserves a little more gratitude from my side, and if possible I'd like to add the user as a co-author of the paper (obviously, after asking for consent). But, given this is my first research paper, I'm not sure what the process would entail.
Would I face any ethical problems in case I send the paper for publication (since the SO user was obviously not "physically" present with us while I was working on the paper)?
Also, do most journals ask for "email consent" from each and every co-author, before publication?