hole crack inside window structure

I'd be taking off as much of that loose material and assessing the size of the gap at that point. Often those windows weren't installed with expanding foam between the frame and the reveal, but you might not have much room to install it at this point. Bu who knows until it's exposed.

Best pick it all out and if it's substantial, then add small buildups of expanding foam then fill the remainder with a quick-dry filler, etc.