I actually text 2 more people which are knowledgeable on the forum. It just i am long time on this forum . And i know they are great advice people. Unfortunately they both not log in now. So i would still continue to change the water and salt.
I am really worry about the girl since they been in the same tank i am pretty sure she will get them or got them.
So let's wait for response and see. And i still want to put for you what OFL wrote. I am not experiences with gills flukes at all, had no clue.
Also don't worry about water changes. I do 100% water changes for my boys all the time. My youngest betta about 2.8 years with 100% water changes. And i had betta that i bought for someone, and i had him in his little cup for 4 weeks with daily water changes so don't worry about that. The only one think with daily or 100% water changes that you need to keep new water the same temp as his water. So you can take one gall jug and put water conditioner and let it sit for 12-24 hrs for the right temp. I would do 100% water changes for the girl also at this moment until you get advice from another people.
Also i think i already wrote or someone wrote do not share anything between them.
Now i will make a few more post to show you what OFL wrote.