、。
My friend is throwing a party for me tomorrow.
Literal
Tomorrow, friend [subject-が] party [object-を] hold-for-me [explanatory-の].
~てくれる marks the action as a favor done for the speaker's benefit — the friend isn't just throwing a party, they're throwing it for me. The sentence-final の is the explanatory particle common in casual speech (especially female speech), equivalent to のだ without the copula.