I have had this Sansevieria for three years. It had a new rhizome bud about three months ago and I waited for it to get bigger and then remove it from mother plant. In the last two weeks the leaves of mother plant are suddenly getting yellow and dying. What's going on? If the new plant is causing it, does removing it help?
Check for root rot, it looks like your plant got (gets) too much water. The roots will then rot, and without healthy roots the plants dies (that's what your looking at, a dying plant).
Try to remove rotting roots and put it in dry soil for a while with proper drainage. And only water it when the soil is dry.
This plant originally come from dry areas in Africa. It can handle drought much better than wet conditions.
This looks seriously dehydrated. it means the roots become dry and die and the rhizome becomes wrinkled and woody. I think yours has lost its roots in some point for some reason such as over watering and root rot or under watering or even soil PH change.
Looking at the amount of the yellow lives its kinda looks too late, but you might still have a chance to save the mother depending on the condition of the rhizome. if you see the rhizome is soft and mushy, even some part of it, then its rotting (rotten) and the follow the other answer and a fungicide might also help you. but if you see the rhizome is hard but wrinkled then continue reading,
I'd separate the baby ASAP and put it in a small pot (relative to it). it stunt its grows for a while but it gives you the chance to save both of them.
Use new fresh potting mix for both of them and wash the old soil as much as possible without hurting the reminder of the roots.
For the mother plant, try to use a mix without dirt,clay, fine sand or anything that holds moisture for too long like cocopeat or peatmoss. for a while the water should wet the mix and go right through the mix very fast. wood particles with perlite mixed 50 50 sounds perfect.
At this stage extra depth is not good too, something like half the current depth is better. this way you can water more frequent to rehydrate the mother without the risk of over watering and rot.
If you curious you can split a plastic soda bottle and punch many holes at the bottom of it and use it as a pot so you can monitor your root development. it might take a month or so for the mother plant to produce new roots and rehydrate. and you’ll loose some leaves that turned yellow but at the end the plant sends out new healthy leaves from its center.