<I am not a doctor or medical student>
Crossing over can be complicated. I won't speak for my experience. But you can read stories of people who have done so at benzobuddies.org to see the varied experiences.
Or if you don't like online forums, Christy Huff, a cardiologist, has written about her Valium crossover experience. I can't recall where it was published, but it was in national papers. I recall she did a 6 week crossover.
When people cross over, it seems like the equivalencies in real life are quite variable, which is born out also by how the guidelines for equivalency vary by up to a factor of four.
Also diazepam is affected by CYP2C19 metabolization, and there are fairly common polymorphisms that affect this, which don't affect Xanax.
I believe—and it was a long time ago I read about this—that the various benzodiazepines attach to various ligands in varying amounts, which leads some to be more anti-convulsant, more anxiolytic, more hypnotic, more soporific than others in relative amounts.
In short, you could be snowed by what is theoretically an equivalent amount of diazepam, especially at the levels of Xanax the OP's patient wast taking. if you crossed over all at once. Or you could theoretically make a drastic cut depending on which equivalency table you use.
I think that's one reason why people do step-wise crossovers, in that they can learn what is actually an equivalent amount for them.
This shows an example of a cross taper:
(I know it seems like an unofficial site, but the NICE guidelines in the UK refer specifically to that same advice, and I can't link to NICE because they geoblock their site.)
Some people never seem to tolerate crossing over well and do better with a direct taper.
The experiences seem incredibly varied in my experience reading other people's accounts.
I would guess one factor is that while diazepam is fast-acting its effects are somewhat cumulative over time as the metabolites build up. I've noticed in some people's accounts, they've said when they've done an immediate crossover they haven't felt the effect of the diazepam for up to even a week or so, which I would assume (large assumption) is when the metabolites accumulate—but that is beyond me, just a guess.