Presumably, a certified diver is capable of conducting a safe dive with her buddy unsupervised, in the conditions she was certified in. It is hard for me to imagine conditions outside a swimming pool more benign then Bonaire. (I haven't seen a whole lot of current or waves in my 80+ dives in Bonaire.) Nevertheless, if she desires a divemaster in Bonaire they are readily available. What navigation are we talking about? The reef is on your left or your right. Just dive sites with easy entry/exit.
As I stated above, diving in Cozumel is often characterized by strong currents, deeper dives, divemaster-led conga lines through swim-throughs, and a lot of boats moving around where divers surface. Any comfort newer divers are getting from their divemaster escort is largely illusory (regardless of the competence of the divemaster)--these dives are considerably riskier than a typical dive in Bonaire.
By the way, coral is fauna, not flora.