How to tell when a joist is too far gone?

[Cherryburn]

In the process of repairing some water damage in the living room from the bathroom above. This ceiling fell through as the "lath" part of the "lath and plaster" basically no longer existed. Joist evidently has some rot along the bottom where the ceiling was, but also sounds solid most of the way through.

Is this too far gone for treatment to be an option? It maybe wouldn't be the worst job if we had to replace it, it's in an alcove so only about 2m long and it's against the party wall here so we could potentially anchor bolt a bit of timber to that. It only seems to be supporting the end of the floor above.

Video here with sound:

Thanks!

 

[Cherryburn]

Well, an update to this. I figured it was worth at least trying to remove the rotted bits, possibly treat and fill. Realised it's just a non starter, the side we can't see is way worse and the screwdriver goes all the way through in places. Even if there is enough wood there structurally (these are massively oversized), I wouldn't be able to fix to it being so uneven.