We are trying to turn my friend's hair silver.

There are still some blue tones in it from his previous dye job.

So we tried mixing normal white toner (Manic panic brand) with some blue to try to create silver (after reading things on this site)

It isn't really removing the yellow, and it sort of tinting it green. It seems tad more silvery toned where the blue is, but...? I could use some serious help.

Can't really advise without a photo but if it's going green then it sounds like the hair wasn't bleached light enough.

try mixing a purple dye instead of blue. It's less likely to turn the yellowish areas green.