@2ND For clarity's sake I presume. His Senpai is a Senpai to him from a previous workplace/college (I'm forgetting which atm). So technically while she's both of their Senpai, she was his first, and as we only know him by Senpai, Senpai's Senpai would be more accurate instead of just Senpai.
Senpai = the MC here, even if his Senpai now works there and is Senpai to Douki-chan as well