There is nothing to do about that. My smart alec answer is "If I could stop that from rotting, I could stop decks, fences, and siding from rotting (without pressure treatment - which would kill the tree). If I could stop your deck and fence from rotting, you couldn't afford me.
What you do want to do is: examine the extent of the decay. How much strength loss is associated with that? It looks promising that there is good wound response. Poke around with a small stiff rod (or even a screw driver). How far can you get it in? Tap on the wound and around the wound with a mallet. Does it sound hollow or solid? If had me out to do a risk assessment and that wood feels solid and doesn't sound bad with the mallet, I'm probably going home. If it does sound bad, I want to know more. Perhaps using a resistograph (or sonic tomography if you need to thin the wallet a bit) to detect the extent of the decay.
Next, the other big question: if the tree falls, will it hit anything?
