You will need an at least 3 channel mixer (like Behringer offers). After connecting all the units (Helix, mic, signal of partner) as well as your headphone/speakers to it, you have to level the volumes of each according to your needs and taste.
If your partner also want to have a mix, you will need a mixer with min. 1 bus system, so that you e.g. us the bus and our partner the main output of the mixer.