This function is used for bookkeeping in a library. How would you refactor this?

refactor.py

from datetime import datetime
from db import db

AuditLog = []
BorrowCounter = 0

def borrow_book(user, book):
    global AuditLog, BorrowCounter

    if not user or not book:
        return None

    if not user.get("active"):
        raise Exception("User inactive")

    stored_book = db.get_book(book["id"])
    if stored_book.get("copies", 0) <= 0:
        return None

    if user.get("borrowed_count", 0) >= 5:
        return {"error": "LIMIT_REACHED"}

    if user.get("unpaid_fees", 0) > 100:
        raise Exception("Outstanding dues too high")

    if datetime.now().weekday() == 6:
        return None

    db.update_book_copies(
        book["id"],
        stored_book["copies"] - 1
    )

    db.update_user_borrow_count(
        user["id"],
        user.get("borrowed_count", 0) + 1
    )

    if user.get("unpaid_fees", 0) > 0:
        db.update_user_fees(
            user["id"],
            user["unpaid_fees"] + 10
        )

    if stored_book.get("category") == "restricted" and user.get("role") != "admin":
        raise Exception("Access denied")

    BorrowCounter += 1

    AuditLog.append({
        "user": user["id"],
        "book": book["id"],
        "ts": datetime.now(),
        "counter": BorrowCounter
    })

    if stored_book.get("priority"):
        return True

    return {
        "status": "ok",
        "due_days": 14
    }