If you can put them/it outside it might help. Gardenias make terrible houseplants usually. I know, “they are someone else’s plants” and you don’t have a lot of control over that. Can you contact their owner and get some instructions from them? If they are Their ‘babies’ they should have a good idea of their needs. If they are new to their owner, the loss is not so great (maybe the cost?) and you will be forgiven soon.
If you have to have them inside they want lots of light (it sounds like you’ve got that covered) and to always be moist. Try to only let the top 1” get dry, usually this means a good deep soak when you water so it can go a week between waterings. I can’t stress enough how important it is that when you water there should be more than a ‘trickle’ coming out the drain holes. Good luck with the rescue.
