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You may have a few things going on here with your plant.
You may need to repot with fresh soil.
You can check the watering, light, fertilizer and humidity needed against the articles below.

https://www.gardeningknowhow.com/houseplants/anthurium/anthurium-has-drooping-leaves.htm
https://www.gardeningknowhow.com/houseplants/anthurium/anthurium-care.htm
https://www.gardeningknowhow.com/houseplants/anthurium/repotting-anthuriums.htm

It sounds like you mean Anthurium- does this look something like your plant?
https://www.gardeningknowhow.com/houseplants/anthurium/anthurium-watering-instructions.htm

Growing in a glass container with no drainage holes (likely with water at the bottom) is not the best way to grow these plants, but if you're planning to return the plant to your friend, just keep it in the glass container and replenish the water when needed to keep it at the same level.

If you will be keeping the plant, I'd recommend repotting it in a traditional container with a soil mix and drainage holes at the bottom. This will make it healthier in the long run. Here is more information about the soil mix and conditions Anthurium prefers:
https://www.gardeningknowhow.com/houseplants/anthurium/anthurium-care.htm

This could be a fungal issue, mottling is often fungus related. This article may help diagnose it:

http://www.gardeningknowhow.com/plant-problems/disease/plant-leaf-spots.htm

It sounds like your lighting is fine, as anthuriums do not like direct sunlight. They also don't like wet feet, so make sure your not overwatering it.

Humidity is the likely missing link. Think tropical. They don't want soggy soil but increasing humidity may help open those flowers.
A tray of water and pebbles can be placed under the pot. You can also run a small room humidifier near the plant.

https://www.gardeningknowhow.com/houseplants/anthurium/anthurium-care.htm

Examine the plant and look for offshoots that emerge from the ground near the main stem. The shoots grow from the roots of the main plant. In one year, up to 15 can appear from a healthy anthurium.
Using sharp garden clippers, cut the roots about 4 to 5 inches deep to remove the offshoots from the mother plant. These aerial roots are necessary for successful propagation.
Replant the divided shoots in 4- to 6-inch pots that are filled with rich, organic potting soil. Keep the soil moist and place the pots in a humid location with partial sun.
https://www.gardeningknowhow.com/houseplants/anthurium/anthurium-care.htm

It is possible!

https://www.gardeningknowhow.com/houseplants/anthurium/anthurium-care.htm