Ok, don't panic!
Sweetcorn is absolutely fine. Ideally they should have pellets first and if you want to give treats it should be later on in the afternoon, to avoid blocking the crop up.
Sour crop is when the contents of the crop ferment. If she has this, the crop will feel swishy, like it's full of water, and her breath may smell.
An impacted crop is one where the food has impacted so that the crop can't empty. It will feel like there is a 'lump' in the crop.
In both cases, the crop will be suspiciously full in the morning, when it should have emptied overnight. But a lump could signify impacted crop, a squishy crop could be sour crop.
To treat;
Sour crop - a vet can prescribe Nystatin which is an anti-fungal medicine, and possibly baytril which is an antibiotic, if it is a bacterial infection causing it. Live yoghurt can also help. You can also 'flush' the crop - if you sort of hold her with her head downwards and massage the crop, some of the liquid may come out.
Impacted crop - again, massaging the contents may break it up a bit, you can also give the bird olive oil (syringe it in) to help the contents slip down. Otherwise a vet can operate.
Seems strange advice from the breeder - have you only just got the bird? Which breeder was it? Most places offer a few weeks 'guarantee'.