As an oboe player, I would first check that the two small pads on the upper joint between the B and A and A and G keys are actually sealing. This is one of the first culprits. 2nd, check the connection between the top key (F#) on the lower joint and the key is touches when joints are together (G key). Have the student play a low D — while playing low D, wiggle the LH Ab key. If there is any change of sound/pitch, this key needs to have a cork put on it for the connector bar to “hold down”.
With oboe, it is rarely a key height issue (sometimes it is, but I doubt in this case) and almost always (99.9% of the time) smaller keys not closing all the way due to adjustment screws.
Hope this helps.