What can I use in Thai curries instead of cilantro or coriander root? (Allergy) I'm trying to make panang curry from scratch for my sister, who is allergic to cilantro (AKA coriander outside of the US). This is not the gene that causes it to taste bad - she cannot eat any part of the plant even in small amounts.

Many Thai curries call for cilantro root. This isn't something I've tasted by itself (though I hear it's sort of similar to the stems of the plant), and I'm not very good at pinpointing specific flavors to try to come up with alternatives anyway :)

What other herb or seasoning could I use to get a similar flavor? 

If you can, please be specific in terms of ratios - many substitutions say "try a little of X with a bit of Y," but I don't know what the flavor I'm going for is so I can't taste-test... I would love to know how much of what substance I could use to replace e.g. 1 tablespoon chopped coriander root.
Just leaving it out is unlikely to result in a disappointing curry - some commercially sold panang pastes omit the coriander root too. What they do not usually omit, though, is the ground coriander seed that is common in all except gaeng kua paste (which is the simplest form of red curry paste, has no dry spices at all, and makes for a great curry still) and some specialty pastes.

Some recipes suggest adding "ginseng" - what they likely mean is krachai/fingerroot, which indeed has a "medicinal" taste that could make a good addition to a coriander-less paste.

Avoid a "broken window" effect to the flavor profile - don't make more "convenient" substitutions (eg using ginger in a red curry paste like this one, using onions for shallots, or using plain lime zest without also adding some kaffir lime leaf.), so the flavor profile is left as intact as you can leave it. Maybe you could also roast your own peanuts freshly. Long cooking time, topping with red bell pepper and chiffonaded kaffir lime leaf - get the other elements done really well.