An assignment is where you sign a purchase contract with a seller and then sell your position in that contract to an end buyer, who closes with the seller instead of you. You never take title. The paperwork is the assignment agreement plus, usually, a look at the original purchase contract to check that it's actually assignable and that the deposit and deadlines still work once a new buyer steps in.
$500 flat for that is inside the normal range. Flat fees for a defined document task commonly sit somewhere between $350 and $900. Hourly work, which is what you'll get for anything with a wrinkle in it, tends to run a few hundred dollars an hour and up depending on the market and the attorney's specialty. If your deal turns into a dispute with the seller, hourly is the number that matters, and it stops being a $500 conversation fast.
The part that catches people new to this: the attorney sitting at the closing table isn't automatically your attorney. In a lot of transactions that person represents the lender, or acts as a neutral settlement agent, or represents the other side. You have your own lawyer when you sign an engagement letter saying what they're doing for you and what it costs. Ask for one in writing before the work starts.
Also worth asking your own state bar rather than your friend: some states require an attorney to conduct or supervise a closing, and some states have rules about how assignment and marketing of a contract can be done. That varies state by state and it's the sort of thing where a rule that applies to him may not apply to you.
