1. As I said, your friend didn't take the time to do his own fish compatibility research. His ignorance is what killed the fish, I have NEVER heard of a fish store that sincerely had the fishes best interest in mind while trying to sell them to you. I don't take any advice from a salesperson as fact, especially when living things are involved.
2. Your levels may be fine at the moment, you just added the fish a couple days ago, but they will soon be spiking with all those fish in there. I think your GBRs will be the first to go, they aren't really beginners fish. Followed by the angel. Maybe your severum will live through the cycle, I don't know much about them. Your pleco doesn't have any algae to eat unless you are feeding him since this is a new tank with no algae. Either way, the severum will get large and messy, and will severely limit any other fish you can put in there with him since he might try to snack on them.
These fish are depending on you guys for their very lives, and you are just playing games? Surely your mother is an adult and if you show her some information on fish tanks she would come to her senses. I thought this was your tank to take care of? If it isn't yours, get your mother on here ASAP so she can learn a thing or two. It's so disappointing to waste your time trying to teach someone about fishlessly cycling and what fish would go well together, just to have them come back and say all the random fish they bought without bothering to cycle the tank. It's very frustrating and disappointing, why bother doing the research on here in the first place just to ignore everything we said?