Mine is a 13" MBA from 2013 with 512 GB SSD, 8 RAM GB and Core i7, which I had ordered as a CTO.
Now, I have never heard of a MBA with 16 GB RAM, but it is perfectly possible that they exist.
Personally, I would be a little concerned at a $900 price for a three year old computer - a computer that cannot be covered by AppleCare; at two years old, I would have been more comfortable with such a price.
Advantages: There is a significant different in the spec - and battery life - between a 2012 and a 2013 MBA - the 2013 is an incomparably better computer, and I will readily get a full day's charge from my battery.
The 2013 (and later) models of the MBA are possibly the best bang for buck - in terms of intersecting at that sweet spot of power, portability, price and battery life that Apple have ever designed.
Disadvantages: Personally, I would have reservations about paying $900 for a three year old computer. Check prices in your area, - r, online for a model of a similar age with - roughly - similar specs.
However, I would be surprised if you could not buy a base model of the MBA, as a refurb, new (with all of the warranties attendant upon that, and the possibility of adding three years of AppleCare to it) for a little more, certainly for around $1,000, or $1,100.
Oh, a final piece of advice: Again, personally, I am reluctant to get into financial contracts with friends. Commercial transactions are one thing, and friends are another, and I, personally, prefer to keep them separate.
This is partly because - if it is a friend - you may expect to be offered the computer for less than the going market rate, and the price you have mentioned does not strike me as below the market rate.
Secondly, if anything goes wrong, it may be hard for you to challenge this, or have some sort of comeback (not without risking the friendship).