I am hoping this is the proper category. My friend has owned here house for just shy of two years. The previous owner had the in ground oil tank decommissioned and that was listed on the form 17. So we are digging up the yard near the old tank and find an uncapped 2" Galv pipe just below the grade. I can see liquid in the bottom and when I dip something in, it smells of petroleum.

So we have no clue what this is. I thought maybe a vent, but we are being told it may be a second tank.

If it is a second tank, it will need to be decommissioned. I have a few questions for anyone who may have experience with these things... if it is a second tank (right next to the first one), is this something that the tank removal company would have missed? How common is it to miss a second tank? How about the inspector? Should someone have picked up on this? I am guessing I am wrong, but it doesn't seem right that my friend should have to pay when the sale was based on the tank(s) being decommissioned.

Any info would be appreciated...

If you can find out who the company was that filled the oil tank they would know from their files whether they filled a second tank. If one may have filled when the other one was topped, then they might tell you if they filled an order for more oil than the one tank could hold. In other words, if the tank holds 200 gallons and they filled an order and it came to 300 gallons, then you would certainly know there must have been a second tank to accommodate the extra 100 gallons.

Sometimes, if there is sufficient oil in a tank that can be reclaimed... that is, used, they might remove the tank at not cost for the amount of oil in it.
If the oil though is just sitting in a pipe that was not removed with the old tank, then you may as well dig around it and find out if its attached to anything.