Correct grammar? 她对待客人很友好。
The first sentence is correct. You don't need a "得" in this sentence.

Here I want to show you a way to maintain your express correct and beautiful.

The key point you can check your gramma in an easy way is detecting the main branch of a sentence and then do some extensions.

For instance:


  The main branch of "Her treatment of the guests is very
  friendly." is "Her treatment is friendly.(她的招待是友好的。)".


After clarify the main branch of a sentence, you can now add the details into your sentence, it's always a best way.

The sentence now looks like: 


  "Her treatment of the guests is very
  friendly.(她对客人的 招待是 很 友好的)"


At last, change and delete some words, which makes your expression more fluent and direct.


  "她对客人的 招待是 很 友好的" -> "她招待客人很友好"


Step by step, a correct and beautiful Chinese sentence is created by yourself.Is it interesting?

Hope my answer helps you.