disease spread to my friends fish

hi so im coming on here to ask what step to take.

basically we went to petco, she got a guppy and a molly, they seemed fine on the way home, got acclimated fine and seemed okay. the next morning the guppy was stuck in the filter, she has other guppies and this hasn’t happened before. after that the guppy weakens and dies. her molly was upside down on the bottom of the tank and passed away yesterday. since then all her fish have died. her betta along with other guppies and plecco have all passed away, she had a loach who is bleeding internally and recently died tonight. the rest of the fish were not from petco, only those two. genuine question is there anything we can do about this? thank you