When Dahlia joined the team, she was with Spencer and JJ when they were going after Tobias Hankel.
Dahlia made the decision to go with Spencer when they split from JJ. This decision resulted in both Dahlia and Spencer being kidnapped by Hankel.

Dahlia managed to leave a hair pin behind for JJ and the others to find.
While her and Spencer were held captive, Dahlia did her best to keep sane, bit similarly to how Spencer was, she wasn't okay afterwards.

Even though Dahlia wasn't drugged as much as Spencer, she still developed a slight addiction, but Dahlia was more so plagued by nightmares.

Dahlia and Spencer are definitely academic rivals to lovers, but this situation did bring them closer as they both knew how to help one another.
(I say academic rivals in the sense that both of them are stupidly smart).