Convention to denote author who left the research group?
From your question, I am concerned that you have not talked to the "author" about the manuscript. You must get permission from all authors prior to submitting a manuscript. It is not acceptable to submit a manuscript without permission from all authors nor is it acceptable to leave an author off just because they have left the group.

As for what is acceptable for an email address and affiliation, almost anything goes (as long as it is not misleading). I personally think universities and research groups should give permanent forwarding addresses to eliminate/reduce these issues.

In terms of the affliation, you should also ask the author. It is not uncommon to list an affiliation as the company/university where the work was done and provide a new corresponding address. As for email, it is common to list only the current address. This might be a new corporate email address, assuming it is appropriate. If not, a personal email or a new personal/professional email address is fine. Again, just ask the author and do what they want.