The Disciple Making Course talks about there being an insider-outsider spectrum. It would be odd for someone of a completely different ethnicity, religion, socio-economic status, language accent to go into certain homes, neighborhoods, or villages.
While both Mohammed and Ahmed are both Tunisian, Mohammed decides they are still pretty different in socio-economic status. Mohammed realizes that he uses a lot more French words when talking to Ahmed since he grew up in France. Mohammed also has been a believer for 5 years, and it would be better to equip Ahmed to do Discovery Bible Studies without Mohammed even being there, so he chooses to model a lesson with Ahmed first.