fungus isn't a quick killer, but many thing that look like fungus are. If it really is fungus, methylene blue could save her, but fungus is unusual on undamaged fish. If the tetra or a male nipped her, or she bashed herself on the cage, fungus could grow on the wounds. Velvet and columnaris look like fungus but need anti-biotics.
The first thing to do is isolate her in clean water and slowly ramp up salt levels. A 5 gallon hospital would be great. True fungus doesn't like salt and platys do. If it stops getting worse, just keep the water clean and the salt level steady until the fish is all better. If it keeps spreading or started on the mouth, assume columnaris and buy something that treats it.
Pregnancy is stressful. It could trigger an outbreak of something the fish was carrying. Some meds are harmful to developing fry, but the mother has to be the priority. Cull visibly deformed fry and start over.