I'm a starcher from way back. I have had that happen a few times. Here is what has caused it to happen to me.
1. The starch mixture was older than 1 week. When I mix my own using tap water , if I let the mixture age more than one week or so it will have disscoloration. This comes from a few causes. The water that comes from the tap still has some impurties , and clorine is used to kill bacteria , but it does not kill all the bacteria . Even the bleach commercials say kills 99percent. So there are some minor amounts that will breed and grow in a homemade starch solution. This can cause the yellowing. Refigerating a homemade solution , even when you dilute a concentrate prolongs the shelf life but it should still be discarded after about 10 days.
2. The starch burns faster than the fabric. Once the starch has a bit of scorch it will wash out, as the starch is soluable .
3. If it is from a commercially prepared spray starch ( in a can) it is propably not a shelf life issue, as they use sterile water in their processes. It would take years to have the solution degrade.
4. If you are using a homemade solution either from a concentrate or scratch, the stronger the solution the more likely it is to happen.
5. If you are using a homemade solution or home diluted solution use a clear bottle. You can see a bit of yellowing starting from age.
If you are concerned about fabrics yellowing on the shelf that have been starched , that is only a concern if there is High moisture content, as water/humidity are the key .
She can spot remove the yellowing with clear water, do note she may see abit of the yellowing spread at first , but keep diluting rinsing the area, till all of the starch has been removed, or wait and launder the final project.