The luminous components of galaxies can act as powerful probes of halo gas in foreground galaxies along the sightline. We use Keck/LRIS absorption-line spectroscopy of a luminous, blue background galaxy at z = 0.69 to study Mg II halo gas in the outskirts (ρ = 11.1h−1kpc) of a massive, poststarburst galaxy in the foreground at z = 0.47. The foreground absorber shows signs of recent merger activity and is host to a low-luminosity AGN. The halo absorption we observe is extremely strong (Wr(2796) = 4.0 ± 0.1Å) and is indicative of a large Mg II gas velocity width (> 400kms−1). We briefly discuss the possible origins of this absorption, including multiphase cooling of hot halo gas and galactic winds/outflows.