If the Arduino pin is an input and is switching reliably with the Pi's 3V3 logic high, there is no problem in normal use. Logic inputs have high impedance.
There is a potential problem at power-up, when the Arduino could be in an indeterminate state -- which could output 5V at low impedance. A series resistor will prevent that doing short term damage (but is not a cure in the long term).
Note that with a direct connection (no resistor) the Arduino is also at risk at startup if both sides are trying to assert different output voltages. While the GPIOs should start up safely (both sides), the protection is for when the code goes wrong. A voltage divider (say 22K from Arduino to Pi, 33K from Pi to ground) will be safe even if the Arduino code wrongly programs an output on that pin. Those values should not affect the Arduino's input response, since the input impedance will be much higher and the 33K to ground will not overload the Pi. YMMV.