If you can afford it, and can also afford to be flexible on repayment periods, I would do it. My parents did this for me and DH, when we needed a car but couldn't get the finance, and we set up a monthly standing order. We got a lump sum from some extra work DH did, and used that to clear the balance early. My parents have also helped my sister with her car, they bought it outright and she pays back monthly. They worked on the basis that if for whatever reason either of us couldn't afford to repay, they would rather us owe them than a finance company. If they had been unable to afford it though, we wouldn't have asked.
If your DS has come up with a business case for the car - cost of car, how ongoing expenses will be met, repayment period, when the balance will be cleared - then I would absolutely consider it in the context of my finances.
DH bought DS a car as his inheritance, but DS pays the insurance, tax, etc. It is an expensive business - even with a black box, and 2 years driving experience with no claims, his latest quote was £1100 for a 5 year old Corsa that does 8000 miles per year.