SCT Controller providing OJT described near loss of separation event when Developmental Controller was slow to react to a developing loss of separation.

Narrative

I was training a low time developmental on West Sector. Aircraft X; a C172; departed MYF for SDM IFR and requested a GPS 8L approach to SDM. The developmental was unsure how to route Aircraft X since there is no flow from West to South Bay on GPS 8L approaches to SDM in the SOP. I stated I usually run them over MZB at 4000. The developmental initially issued a climb to 4000; however failed to realize there was another IFR Cessna coming northbound over MZB at 4000. I brought it to the developmental's attention and stated to get a point out from North Sector on the northbound traffic and climb it to 6000. However; due to traffic in the North Sector's sky; the point out was not able to be approved. I then told the developmental to stop Aircraft X at 3000 and APREQ 3000 with the South Bay Sector; which was approved. The developmental issued Aircraft X a left turn direct MZB; however; since the aircraft was already NW of MZB; that was going to put Aircraft X into a potential conflict with a B737 departing SAN on the PEBLE departure which would climb to 15;000. I then instructed the developmental to turn Aircraft X to a 090 heading to get the aircraft directly north of MZB; before turning direct to MZB; which he/she did. When Aircraft X was 2 miles directly north of MZB; the developmental asked if he/she could turn the aircraft direct to MZB; and I said yes; and the developmental did so. At the same time; the South Bay controller called us to state to keep an eye out on the northbound traffic at 4000; since the pilot was having a hard time complying with control instructions. At this point; another CPC came to relieve us on West Sector. During the brief; the relieving controller asked if the SAN departures were stopped at a lower altitude; and I stated no; as I had projected (based on my view of the radar from the left side of the developmental) that Aircraft X was going to pass behind the B737 off SAN. At the same time; I noticed the northbound aircraft at 4000 had drifted slightly west of MZB; and the B737 checked in so I stopped the B737 at 3000. The relieving controller again expressed concern about the separation with the B737 and Aircraft X; but I believed I had 3 miles lateral separation; as well as diverging headings. The relieving controller then placed a 3 mile ring around the B737; and Aircraft X was within the ring. At that time; I gave Aircraft X another turn to a 090 heading; to further increase the separation with the B737; and climbed the B737 to 15;000. Recommendation; add a flow in the SOP from West Sector to South Bay sector; on aircraft departing MYF for the Visual or GPS 8L approach to SDM airport.