From Halopedia, the Halo wiki
When the Pillar of Autumn fled during the Battle of Reach and found itself at Installation 04, Valdez persisted in his duties tending the wounded and dead even as the Covenant began to board the ship. As the fight progressed he, together with the remaining ODSTs, sought to defend the engine room where his medical skills were used to treat First Lieutenant Melissa McKay for a plasma burn on her arm.
Valdez' fate is unknown as is whether he died aboard the Pillar of Autumn or evacuated to the surface of the Installation, as he is not mentioned afterward. His death is confirmed when either the captured Truth and Reconciliation was destroyed or when John-117 destroyed Halo.
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