Will an arduino output pin set low ground a pin on a DFPlayer module
Two simple methods.


A NPN Transistor across the push button. A 1k Base resistor is good enough, as you are using this as a switch not as a current multiplier.




Output High to press the button. Output Low to release.


Since you are sharing grounds, and there is a resistor involved, you could just connect it in a "Open-Collector" setup. You switch your pin from Input to Output Low. Never Output High. This is like the bit-banged i2c setups. Not Input with Pull-up.




Output Low to press the button. Input to release.