It is "take HER serious" (as in, take serious what she says) or "TAKE her seriously" (as in, give her a real good time).
When you "take HER serious", serious says something about how YOU take HER and "her" is a noun, therefore you need to use an adjective, in this case "serious".
When you "TAKE her seriously", seriously says something about how YOU TAKE her and "take" is a verb, therefore you need to use an adverb, in this case "seriously".
An easy way to remember:
You can use both in one sentence, when you're seriously (adverb) not taking her serious (adjective).
PS: Saying that you never hear people say "take serious", so therefore it can't be right, says more about your environment than about if it's right or not.