You may want to consider using another recommender. HBS wants the employers’ email address to match the company listed that they’re working at, and they also want to see a professional company email from your recommender. It’s one of the ways they validate the legitimacy; if it’s @gmail.com there’s no way to know whether they were ever your supervisor.
If you can’t perhaps it’s worth putting the old job title down and clarifying this in the optional essay.
Posted from my mobile device