LED Pin 13 Input or Output?
Like any other I/O pin, there should be no problem driving this pin (and
thus the builtin LED) from an external source, as long as you make sure
it is in INPUT mode. If the pin is set to OUTPUT you will likely
damage the Arduino.

Warning: If you have the Blink example already loaded in the
Arduino, the pin is in output mode. Be sure to upload another program,
that does not set pin 13 to output, before connecting anything that
could drive pin 13.