Looks ike the ones I have that I took in for the winter. Could be too much water, but I've seen it happen even when the water is fine. So I don't know. Maybe it is lack of light? That is one other factor I can think of or lack of humidity. I noticed better luck bagging cuttings - to a degree.

But I notice a high tendency to grow mold all over the leaves, which isn't good. And doesn't sound right. I used to treat begonias with that kind of mold with greenlight fungaway (I still have a bottle) but can't use it until i buy a new spray bottle to dilute the solution in. That helped some, but not entirely. I don't know if it will work on my impatines. I'm trying another solution, not systemic, but treats a broad range.

But even still mine, looking like your plant, eventually die. I suspect a root or stem rot pathogen. So I am looking for help like you.

You will never kill busy lizzie with overwatering. Here in Brazil it is often found in streamside/bog conditions. I'm puzzled why one is dead and the other healthy. I suspect some kind of root problem. Repot the healthy one.

I hate to disagree but you can kill impatiens with too much water when they are grown in pots or containers. If they are in the ground it is a different matter. The water is disbursed in the ground even in a streamside the soil is not overly wet. In pots the water is pooled around the roots and that can kill most plants.

If the plant has root rot from overwatering there may be no saving it and the root rot may spread to the other plant. I would remove the healthy one and repot it and then place unwet soil in the space you removed it from. Some of the moisture will wick into the unwet soil. Then place the pot where it gets good ventilation and more light and don't water until the soil is bone dry. If the roots have not started to rot it will come back. Then watch the water. Plants inside only need about half the water indoors than they need in the sun and wind outside

It seems like the water should have come out of the other holes already (not just the new ones you made) so yes, whatever other factors there are (including weak plant), water seems to have played a part. Definitely can move it out to another pot, otherwise you risk the healthy plant keeling over in 24 or 48 hours, etc while it tries to live in that condition.

However, I think you don't have to repot into a larger one . You could just remove the soggy soil and put dry soil into that same pot. Clean up the root ball as much as possible from the other plant's roots and then insert the new root ball back into the same pot with new, dry soil.

Let the soil moisture leech out from there to the dry soil.

And you should also clean up those dead leaves. It will help with air circulation and make the pot look better.

Certain impatients are more prone than others to this dampness in the stalk when leads to rot ..repot the healthy plant with a generous layer of gravel in the base of the pot..keep in the sunlight and away from cold air which also contributes to this rot issue ..water sparingly in the off season over winter plants indoors...or try one of the old fashioned variety which are a much stronger and more resilient plant...

I don't have much luck keeping NG impatiens indoors, unless they are covered in a dome or in a bag. The one I tried to overwinter this year didn't make it. They seem too susceptible to fungal diseases if kept humid and moist. Yet if you keep it drier, then they start wilting and dropping leaves for lack of something. I've yet to see them happy indoors, though I wish. They'd make an amazing houseplant if only I knew what it wanted and could provide...

That said, maybe it is a lighting issue. When I was more successful I had it directly under the lights with my other plants, though bagged. (Even then it remained tiny and lanky, so even the fluorescent light may not have been enough.) When it died was when I took it away from that area. (I was concerned about viruses and any thrips.)

Outdoors: In my shade garden they performed, though didn't bloom as much as the ones kept in the full sun back area. (those received over 6+ hours of full sun.) The sun ones (all were kept in containers) performed well but were constantly wilting for lack of water, even if given daily.

N.G. impatiens seem to demand more light than regular ones. My guess to grow them indoors long term would require a high intensity light or something of that format. *sigh* a bit out of my budget to buy or maintain.