Hi. Usually you get 2 recommendations (though some schools have moved down to 1) due to these situations happening fairly frequently where you worked a year or two with one supervisor and then 6 months with another. I think it is totally fine to have this individual submit a rec letter for you with their gmail address. They can outline some of the details of their transition in the section of the LOR where it asks about "your relationship to the applicant"
Here is what Ross's standard/guideline is:
Quote:
Michigan Ross uses the GMAC Common Recommendation Form, and we require only one recommendation. Select a recommender who can discuss your professional performance and work style; a current or former supervisor is ideal.
https://michiganross.umich.edu/graduate ... quirementsYou can also use the optional essay to write a short paragraph explaining this situation to the adcom if that's all explaining you have to do
Good luck with the applications!
P.S. Your profile shows you got admitted to ISB earlier? You not going?