Apple seeds contain a compound called amygdalin which can break down in the stomach to release cyanide and as we all know, cyanide can kill but like most things, it is the dose that matters. If your bird eats enough apple seeds then yes, apples seeds can kill your bird, although as that is said, it would take many, many seeds to do any harm to our birds.(around 50 Apple seeds to harm a bird that weighs 100g)
Apples themselves, without the seeds or including the pit, is safe. Feeding too much of any fruit can lead to a fat bird which can cause health problems. Since your bird likes apples, use that opportunity to train her or have her forage than to just place the fruit in a bowl.