sagging ceiling

I recently put an oil tank in basement. The installers had to drill through the house to the outside wall in order to fit the fill and vent pipes. While drilling the vent hole, they came up against a wall stud. They elected to drill through it to fit the pipe to the ouside of the house, My question is this...will it effect the structure of the house? I know it probably is not the best practice, but is it safe to 'remove' one stud from the wall without hurting anything?

I'm thinking about knocking out a part of the wall between the kitchen and living room. As you are in the living room it is a solid wall. As you walk around to the kitchen there is a closet. In the kitchen, attached to the backside of the closet is a banister and the basement stairs are along the other side of the living room wall and you are looking at the back of the closet as you walk down the stairs. I believe the closet is the main load bearing point and I'm hoping I can cut out a "window" on the wall. I'm concerned about the stability of the lower portion of the wall once removed. There is a railing to the basement that would still be attached to the wall. I would hate to cut out the wall and have someone lean to hard on the railing and pull the wall down. Hope all of this makes sense.