•Symptoms: One or both eyes will swell and bulge out. It in itself can also be a symptom of Dropsy.
•Treatment: Usually not fatal if treated, but your betta might lose an eye. Conservative: If the swelling is extreme then use ES (Epsom Salt) first. Perform 100% daily water changes. You may be able to treat the popeye with just Epsom and clean water. However, if the swelling goes down but the eyes remain cloudy/white, then switch to Aq.Salt at 1tsp/gal or use medications. Do not combine Aq.Salt with medications but you can continue to use ES during medical treatments. Medication: If ES or Aq.Salt do not do the trick or it is a serious case of popeye, then combine the ES with API Tetracycline, API Fungus Cure, API General Cure, API Triple Sulfa, OR Mardel’s Maracyn.
The other disease your "expert friend" may be talking about with your second betta is fin rot, and that can/will kill your fish in time. Can't be sure without pics, but black edges are a sign. Either that or he's just nibbling his own fins, which some do.
Here is something that is helpful when requesting help. Makes it easier to diagnose or make suggestions on prevention. Sorry, I know you went through the trouble of typing all that out. It's a sticky on top of this section if you need reference.
What size is your tank?
What temperature is your tank?
Does your tank have a filter?
Does your tank have an air stone or other type of aeration?
Is your tank heated?
What tank mates does your betta fish live with?
What type of food do you feed your betta fish?
How often do you feed your betta fish?
How often do you perform a water change?
What percentage of the water do you change when you perform a water change?
What type of additives do you add to the water when you perform a water change?
Have you tested your water? If so, what are the following parameters?
Symptoms and Treatment
How has your betta fish's appearance changed?
How has your betta fish's behavior changed?
When did you start noticing the symptoms?
Have you started treating your fish? If so, how?
Does your fish have any history of being ill?
How old is your fish (approximately)?
And yes, a betta is worth it lol.