[mojave 10.14.6] How to record headphones mic only ?

Hi there, I searched for an hour for this, and can’t find anything that works, so here I am.

Context : I want to record a podcast with a friend on discord. She will record her own audio on audacity, and I will record mine. After a few tests :
no problem for me (I’m on windows tho). But for her, we can’t find a way to record only her headphone’s mic.

Problem : she dosen’t have the choice to record her headphone’s mic AND internal audio at the same time (the input options for her are Core Audio / Mic/Aux). She’s using a generic pair of headphones with built in mic (the handsfree earbuds type), plugged in minijack into her macbook. We want mic input only, not the internal audio at the same time.

Thanks in advance