DAL controller providing OJT failed to correct developmentals altitude assignment below MVA. The reporter alleged that D10 should have resolved potential conflict prior to hand off and that instructor intervention should have occurred.

Narrative

DAL was in a North flow; Runway 31L/R configuration. I was training a CPC-IT on the Local Control position. We had a Single Engine; VFR; inbound from the Southeast to land Runway 31R. NOTE: DAL has a 2400 foot MVA on final ranging basically from the approach end of the runways to six mile southeast of the airport. After the Single Engine was talking to us; DAL-S at D-10; who works all of our arrivals into DAL; called up and inquired if we could provide visual separation between the Single Engine; who was five miles southeast of the airport and a B-737 who was on a twelve mile final to Runway 31L. The trainee approved the request. DAL-S didn't switch the B-737 to us until he was inside of three miles from the Single Engine. The trainee wanted to get the Single Engine to descend so he told the Single Engine to descend to 2000; I thought he said through 2000; which he should have been doing anyway. The trainee was doing this to try the get the Single Engine under the inbound B737. When we did get the B-737 on frequency; the trainee did call the traffic and the Single Engine had started a descent through 1900. I feel the Single Engine did not think it was an altitude assignment. According to our facility QA department; when the trainee told the Single Engine to descend to 2000; they consider this an advisory assignment and according to 7-9-7 in the 7110.65; you can not assign or advise an aircraft an altitude below the MVA. Recommend that we should not have taken on the responsibility of the separation between the two arrivals. DAL-S handed us a bad situation to begin with. We should have made them clean up the mess; so the trainee would not have felt compelled to descend the Single Engine like he/she did. Additional classroom is needed for pre-briefing of MVA minimums with the trainee so they understand what advisory means in 7-9-7 in the 7110.65. Most important; I; as the trainer; should have deleted any altitude for the Single Engine that could be perceived as an altitude restriction or assignment.