Leases never require any type of capitalized cost reduction (down payment). They may be advertised with one as a way to achieve a certain payment, but they are not required. Typically the inception money on a lease will consist of the first payment, security deposit, bank or acquisition fee and registration fees. If desired, some or all of these can be rolled into the capitalized cost and paid in the lease payment, but you end up paying interest on these.
As for short term leases, most leasing companies will lease from terms between 12 and 60 months. However, because a lease payment is partially based on the amount of depreciation and depreciation in the first couple of years is less than it is over longer terms, shorter term lease payments tend to be very expensive.
For example, let's say a vehicle has a 24 month residual of 69% and a 36 month residual is 59%. Say the MSRP is $35K. 69% of 35K is $24150, which means that you would be paying depreciation of $10,850 over 24 months, or about $452/mo. The 59% residual is $20,650, so you would be paying depreciation of $14,350, but over 36 months or about $399/mo which is over $50/mo less than the 24 month payment. However, keep in mind that these hypothetical lease payments don't include interest.
The long and short of it is that a short term lease, such as an 18 month lease, would be very expensive when it comes to monthly payments. Of course, this doesn't even address the fact that if your friend's friend is coming from a different country, he won't have a social security number or a US credit background, which will make getting any type of loan difficult, if not impossible.