Air carrier pilot reported while on final approach ATC advised them of a helicopter that was maintaining visual separation from them; but the pilot had to reduce the descent rate to avoid a collision with the helicopter.

Narrative

While shooting the Visual RWY 35 into PHL; Tower gave our location and asked an Army helicopter flying north along the river if if they had us in sight. The Army helicopter stated they had us in sight and were asked to maintain visual separation. The CA and I both established traffic in sight as well. As we continued the approach; we figured that we would be crossing paths if we continued with current rate of descent. To assure visual separation; the CA reduced the rate of descent momentarily - briefly appearing high on the visual approach. This whole time we also had the helicopter on our TCAS; as it passed under us; it showed 200' below us. Once the helicopter was no longer a factor; the approach was continued to a successful and safe landing. Once off the RWY; the CA proceeded to let Tower know about the separation given and how it was very close.Cause: Aircraft crossing paths while on approach; needing to ensure proper visual separation.Suggestion: Crews operating along the river corridors must maintain heightened vigilance while adhering to appropriate altitude assignments.