High and Low Potassium Foods?

Just asking for my mothers friend who has a problem regarding potassium production, apparently his body is producing too much potassium and they can’t operate yet for whatever reason

I’ve tried google and searching but not too much seems to come up, the individual is booked to see a dietitian in a fortnight but the situation is apparently quite serious and they want to lower the levels asap.

Does anyone know of any high level potassium foods which should be avoided, all i could think of was bananas and nuts.

Also any low level potassium foods?

Sorry if this makes little sense my mum just asked to try find some high and low level potassium foods so she could pass it on.