Cross-Platform Co-op in Call of Duty: Modern Warfare 2 (2009)?

No. The original Modern Warfare 2 (2009) does not support cross-platform play. PC, PlayStation 3, and Xbox 360 players are segregated into separate online ecosystems.

Co-op modes (Spec Ops) are only available between players on the same platform.

Note: The 2022 Modern Warfare II reboot introduced full cross-play, but the 2009 title never received this feature.