Probably the easiest way to flip an entire set of nodes is to change the scale of the parent node to a negative number on the axis you wanting to flip. So to flip the player on the horizontal axis, something like this should, in theory, work:
# Flip everything on the X axis
self.scale.x *= -1
# Flip everything on the Y axis
self.scale.y *= -1
The only thing I'm not sure about is whether this will mess things up or not. I think it should work, from what I remember, but it might be negative scales do strange things.
Hopefully this helps!
(Side note: welcome to the forums!)