A lithium ion charger is likely safe for a lithium polymer, but may not charge it completely, and there can be a discharge concern.

The charge process is essentially the same, but many lithium ion chargers terminate at 4.1 Volts per cell instead of the typical 4.2V/cell for lithium polymer. If so the lithium polymer will get about a 85~90% charge. Won't hurt the battery, likely even increase it's life, the amount of cycles, just a little less run time.

Be aware there are some older chargers that go to 4.3V/cell which reduces cell life and can push the edge of safety, best to verify the voltage termination of the charger I think.

Possibly a bigger concern is discharging if the charger has that capability and there is some need to use it. Many lithium ion chargers discharge to 2.5V/cell, way too low for lithium polymer. Lithium polymer should be no lower than 3V/cell, many recommend 3.4V/cell for better cycle life.