I worked with a woman who had this problem; let's just say we all knew when she'd been in the bathroom in the last couple hours. No idea what caused it (save that she seemed clean/normal-smelling when clothed), and she was cis. Thankfully, since I only worked with her, I never had to bring it up! I don't envy you this conversation.
As an aside, no, a bath isn't necessary, but it IS required to spend a little time actually cleaning the area.

Soap on the outside only, if preferred, and *definitely* pull everything apart in the shower and rinse thoroughly. The vagina itself is self-cleaning, but it does this by shedding unwanted material to the entrance - meaning the vulva is NOT going to say clean by itself, and a good rinsing is necessary to get truly rid of everything. Maybe (?) she's not used to the degree of, er, "detailed" cleaning that a vulva requires as opposed to some penises. (Brassard at least says that a post-op vagina becomes as self-cleaning as any other with time - after the first few months - but it will still have the same limitations as well, and therefore the vulva will need the same careful washing.)
Honestly, though, with a smell THAT strong, the most likely culprit is an infection. Most of those are not life-threatening, but still, she doesn't want to live with it forever. Maybe if you phrase this as a concern for her health rather than specifically about "that smell"?