Net and multiplet chemically induced dynamic electron polarization (CIDEP) of the observer/catalyst spin formed in recombination of the radical-biradical pair is studied theoretically. We obtained analytical expressions for the observer spin CIDEP in the high magnetic field and for the multiplet polarization in zero magnetic field. Polarization in the vicinity of the so-called J resonance and its magnetic field dependence are investigated numerically. The observer spin methodology can be useful for probing magnetic interactions in the short-lived spin triads.