It is unrealistic to expect a diver making his 5th dive (first ocean dive) to roll back and go head down to get to the bottom quickly. Give the guy time to come back to the surface after the back roll, gather his wits, make sure that he is ready to go down then descend as a buddy team.
Expect that he may need additional time. Maybe it is a buoyancy issue. Not enough weight. Still learning to clear his ears. Whatever.
I applaud Evelyn for going back to the surface once she became separated from the group. Let's use her dive as a learning experience and we will use Monday morning quarterbacking or hindsight...whichever you prefer...to see what actions might have worked better.
First we have to know that her son was on his fifth dive and first ocean dive. Evelyn had some concerns. I am not sure exactly what. She has a lot of dives but maybe it has been several years since she last dove. First time in Cozumel? Unknown.
Both of these suggest that having a dedicated divemaster (DM)/instructor for the mother/son buddies would have been a good option. Letting the divemaster/instructor know the novice level of the son and any concerns of the mom allows the DM to address things to be covered while on the boat heading to the dive site.
The DM can cover things that they will do during the first dive to get everyone up to dealing with Cozumel currents. They can talk about the descent and how they will go slow as required to make sure that both are weighted properly, ears are clearing OK and address anything else that may come up.
If the private DM is not used, then both should probably be entering the water on the same side of the boat (backroll assumed) so that they can find each other and descend together. Both need to be alert that the other is doing fine and not force the other to descend faster than is acceptable for that person.
Another point to think about. In this case Evelyn came up on her own. What if the dive boat had moved north and was not looking for her? Does she have any signalling devices...mirror, whistle, surface marker buoy (SMB)? How long would she wait for her dive boat or another to come along? At what point do you consider a surface swim to shore or do you just wait? That may be influenced by having a SMB or not.
Hopefully the remaining dives went well and her son is at the start of a lifetime of great diving.