Euphorbia has root rot

I have a euphorbia cactus, (I believe it is known as an African milk tree) that has gotten root rot. I’ve had it for years but a couple of months ago I had to move it to a place where it doesn’t get much light.

I noticed today that the bottom of the cactus is soft and turning brown but that most of it is still firm and green. Is there anything I can do to save this? Is there any possibility of cutting off the bottom and rooting it? I’ve tried many times to root pieces of cactus that broke off and have never had any success.

I understand that the conditions I have has this cactus in are not favorable (soil isn’t draining enough, pot doesn’t have a hole, low light environment) and I plan to fix all that if I can save it.

Any thoughts on saving it?