Simple Mobile? Very interesting, never heard of them prior to today.
It appears their business model is "bring your own unlocked GSM phone, and use our SIM card."
So you said your friend's Vibrant is currently on Simple Mobile, that means his phone is already unlocked
, otherwise the simple mobile sim card would not work in his phone.
If you phrased the question incorrectly
, and you meant he wants to use his Vibrant in the future with a simple mobile sim, and needs an unlock code, then try calling simple mobile directly and see if they will provide him with an unlock code(but it appears they expect you to already have an unlocked phone in advance). Otherwise if he is currently with T-mobile then T-mobile provides customers who have been with t-mobile for 40 days, with free unlock codes. That is the simplest way of getting an unlock code.
To actually use the unlock code, there really is no place for you to enter the code until you stick in a foreign sim card. Meaning, if the phone is a T-Mobile Vibrant, the moment you put in a non-T-mobile sim card, at that moment the phone will promt you to enter an unlock code.
Technically, there is another way to unlock the phone, without being prompted, but it isn't suggested. Most people just put in the foreign sim, get the prompt, and enter the code.
Again, if your friend is already using the simple-mobile sim card successfully in his Vibrant, then the phone is already unlocked, and someone had already entered the unlock code.
To answer some of your various other questions.
You can root the phone many ways, including the way the user above me suggested.
I personally root manually, meaning I download the update.zip file and place the file onto my phones internal 16gb memory. Reboot the phone in recovery mode, and apply the root.
visit this link, it provides the download link also.
xda-developers - View Single Post - Root your Vibrant
Some other things to check.
Go to settings then click "about phone." What does it say under Baseband Version or Build number. Does it end with JFD or JI6? If it ends with JFD you can get the unlock code very easily using "SGS Unlock" app. If it says JI6, then you will have to jump through more loops.
Again, I think your friends phone is already unlocked. If his phone was a T-Mobile Vibrant, and is now using Simple Mobile Sim, then it is already unlocked.