There are some major differences between leasing and financing, but it mainly depends on your personal preferences.
If you're looking at buying a new car with all the fancy bells and whistles, leasing is probably your better option.
When you lease a car, you're only financing the amount of the depreciation that will occur during the term of the lease. USNews has a good example of this:
If you buy an SUV for $30,000, and put 10% down, you would finance $27,000 for that car (if you chose to finance).
If you chose to lease that $30K car, the dealership would predict the future value of the car in, let's say 3 years. They predict that $30K car to be worth $16,500 in three years, which is when your lease would be up.
With the lease, you would only finance the difference between the original price and the predicted future value, meaning you'd only have to be paying off $13,500.
So, the main difference here is that leasing is obtaining financing for the difference in current value versus the future value when your lease is up.
Also, with a lease, there are mileage requirements. So, if your friend will be putting a lot of miles on the car, he might want to look into purchasing it with a loan instead of a lease.
Leases also require you to keep the car in good condition.
A pro of a lease is that dealerships will typically cover any maintenance for the first few years, so that's additional money you'd save.
Unfortunately, leasing a car means you won't own it at the end of the lease.
I'd recommend a lease for anyone who wants the newest and best technology and won't be putting a lot of miles on their car.
If you choose to finance the car through a loan, you're financing the entire price of the car. Using the same example above, you'd have to finance $30K instead of the $13.5K with a lease.
The huge benefits to financing a car is that you'll own it once the loan is paid off, you can put as many miles as you want on it, and you won't have to worry about the condition of the car.
Financing a car can be more expensive than a lease, especially if you're going with a newer car, but it will turn into an asset for you.
I'm a huge supporter of buying used cars with loans because you will have an asset you can later sell or trade in towards a newer car. If you lease a vehicle, you won't own it and therefore can't sell it.
When buying a vehicle, you typically need to have a down payment.
I recommend financing/buying a vehicle if you're looking at older cars that aren't too expensive, will be doing a lot of driving and if you want to own an asset at the end of your loan.
I hope this helps you and your friend make a decision! Again, it really depends on personal needs and financial abilities, but this is some general advice that should help you out.