It's within the mammal world. You have mammals that lay eggs, and mammals that give live birth.
It is irrelevant when some synapsids specialized into stop laying egg, both for reason you mention (i.e. they never stopped lay eggs) but also for the reason that egg laying per se has nothing to do with the fact that a mammal is a synapsid. The question was if there existed a non-mammal that can make mammals, and the answer is yes; the synapsids.
So the question of when synapsids "stopped" lay egg (we never did) and produce live youngs is a
red herring.
The reason this red herring is brought up is because otherwise
justlookinla must confront the uncomfortable conclusion that mammalian synapsids is just specialized synapsids that make more specialized synapsids. Just like Irish setter is a special dog that makes more specialized dogs, you don't need to know anything about fossils in order to know an Irish setters is a dogs that makes more dog, or at a higher level a very, very very specialized synapsid that makes more
very, very very specialized synapsids.
Dogs makes more dogs, mammals makes more mammals. Synapsids makes more synapsids, life makes more life.
That is what life is about; make more of the same.
As a side note, since some mammalian synapsids still lays eggs - where do we draw the line of "when" mammals stopped lay egg? In my opinion all mamalian synapsids still lays egg, since all mammals still
makes eggs.