I'm not sure, but the place Dallas explores may have been stacked horizontal shafts connected with vertical ones. In which case, if Dallas had entered a shaft at one end, then one side would be his back (where he entered) and the other end that he's facing his front. If the same can be made in Lambert's monitor (e.g., the left side of the screen's the part of the shaft where Dallas entered), then she would have told Dallas that the creature is either behind or in front of him. If it's behind, then it's not on the same level, as Dallas would have encountered it right away, unless the vertical shaft in which he finds himself in the scene is a second or subsequent one (which means the creature would still be in another shaft or entered his using one of the vertical ones now behind him).
In either case, Dallas could only face the side where the creature is coming from and wait for it to reach him.
Of course, the situation gets worse if there are also parallel and perpendicular horizontal shafts.