What's your question? Ask

It could be either! They grow, similarly, to orchids. Too moist of a soil, and too much nutrients will be two of the most common causes of the plants' death.

This article will help you to care for the plant, properly:

https://www.gardeningknowhow.com/houseplants/birds-nest-fern/birds-nest-fern-care.htm

If this isn't fed throughout the year, then this is likely the issue. I recommend feeding with a half strength liquid orchid feed two or three times per year. Feeding at full strength is not ideal, so be sure to cut the recipe from the instructions in half when mixing into you water.

Here is an article that will offer more information on the care of this plant:

https://www.gardeningknowhow.com/houseplants/birds-nest-fern/birds-nest-fern-care.htm

The repotting could be the issue if the container was more than one size up. The extra soil actually can cause root rot, a complication of overwatering.

When leaves start to yellow and limp, I immediately think of too much water. You don't mention fertilizer, it may need a fertilizer pick-me-up.

https://www.gardeningknowhow.com/houseplants/birds-nest-fern/birds-nest-fern-care.htm

I would not. I would opt to keep them separate. The growing conditions can overlap, but they are not similar enough to keep them both happy. The Bird's Nest Fern will appreciate a drier, less humid environment than an orchid will. Planting them together, you would need to sacrifice the health of your fern to ensure that the orchid does not dry out.

Here is an article that will help you with the care of the fern:

https://www.gardeningknowhow.com/houseplants/birds-nest-fern/birds-nest-fern-care.htm

This article will help you with orchid care:

https://www.gardeningknowhow.com/ornamental/flowers/orchids/indoor-orchid-care.htm

Clip off the damaged leaves at the base to prevent further spread. You can try a bacteriacide but there is doubt whether it will work. If the damage continues to spread you may need to destroy the plant to prevent infecting other plants if you have them.

I'm a little concerned that planter might be too big to move it into just yet. Does the plant seem rootbound in the 6 inch pot or is it a nursery pot that you'd like in a nicer container? The rule of thumb is only go up one or two sizes when repotting because if there is a lot of excess soil in the container you can end up overwatering and killing the plant.

These are epiphytic plants which mean they usually grow on a host such as a tree or structure. So they need a very well draining soil mix such as peat moss, coarse sand, and perlite.

Here is more information and care of the plant:

https://www.gardeningknowhow.com/houseplants/birds-nest-fern/birds-nest-fern-care.htm