First, the amount of money you are talking about could not possibly be 1/2 of the entire well. Drilling costs are much higher than that. Most likely, what your friend has purchased is a share of the royalty payments. Royalties will, of course only be payed if the well comes in, and even then it will only be a fraction of the total value of the oil produced. The chances of the well actually producing oil will depend on several factors but it could be as low as 1-2%