Friction has no effect on the speed of light. The speed of light is set by the electric permittivity and magnetic permeability, which are both properties of empty space.
Just wondering, who told you that "everything that travels is eventually slowed down by friction"? This isn't true...who told you that? Only macroscopic systems created in our Earth world with practical considerations are slowed down by friction. There is no fundamental law that friction has to act, in fact quite the opposite, it is called Newton's first law. Friction is nothing more than a hidden force, which is just a challenge of which to get rid.
Why does it always travel the same speed?
It just does. All observers measure light to travel at the same speed no matter how fast said observer travels. Maxwell's equations predict that there is a wave which uses electric and magnetic fields to propagate, and that wave is light.
Is there anyway to slow down light?
Sort of. If you are wearing glasses and reading this, your glasses work because you can effectively "slow light down". Notice I used the word "effectively".
Materials which are transparent have the ability to change the effective speed of light within them, such that wavelengths get shorter and the phenomena of refraction occurs. Glass, plastic, water, diamond, and even air to a small degree are optically dense materials which can do this.
BUT, you aren't really slowing actual light down. The molecules of optically dense media function more as "stop signs" to light than the "roadway". Free space and the EM fields between molecules is its "roadway". What really happens during transmission of light is that the transparent media molecules absorb and re-emit light, making it waste time in billions of molecules. It just seems to us at the common level that light is slowed down.
Is there a way to actually slow down light in free space? No there is not. Even gravitational lensing (a phenomena of light bending in extreme gravitational fields) doesn't slow down light, it just red-shifts the light.
How fast would light travel if there was friction affecting its speed? I don't really know how friction would apply a force to light.