To get the color darker, he needs to use a darker stain.

Using a dye over a stain may cause problems long term with age and sun.

Once the stain is dry, a finish needs to be applied. Bona makes several floor finishes. MinWax makes a floor poly that can be applied in three coats(recommended) in 24 hours. There is a serious odor associated with that oil based product as it cures, however. Then 72 hours to furniture and 30 days to area rugs.

The problem with the most common 'stains' is that they are pigment stains, really just thin paint.

The darker it gets the more pigment is required, until the 'stain' is just a coat of paint obscuring any figure the wood might have had.

Even with aniline dye at some point the finish becomes very plain looking.
'Ebonized' wood falls into this group often.
While real Ebony has some figure present, Ebonized wood often becomes an almost uniform black.

I have used analine dye many times and that is my prefered way to stain wood. Analine dye gets darker with each application. Oil based stain does not. The problem is, I don't know how dye would work over oil based stain. It's usually the other way around.
On the other hand, Bona Kemi is a great product to use to finish a floor. They have a stain for floors and you can use that to stain bare wood. They also have an excellent system for applying the finish. Bona Kemi is made for one thing; floors. I believe Bona Kemi Traffic over their stain using their 18inch wand, would give you a professional looking finish.

"The problem is, I don't know how dye would work over oil based stain."

Aniline dye cannot penetrate the oil surface and will not work at all.

Even shellac is enough of a finish to limit dye penetration.

The one potential problem with aniline dye on the floor is the extent to which exposed areas will fade vs. the covered shadowed areas that will not. Quite a few aniline colors fade badly in sunlight, while others are somewhat more stable. When a color is made up of more than one component dye color, "interesting" color shifts are possible as one component bleaches out and another does not. Over the years dyes have been improved, and while it's possible to purchase examples that purport light-fastness, it's a hard way to find to the contrary. Imagine moving a rug a year later to see a very dramatic color change line at the edge.
The clear finish itself can offer a good amount of UV filtering and that component should go into the selection process.
Earth pigment stains are the most color fast of all, which is why they haven't been completely replaced by deeper, clearer dye stains.
I love anilines for a lot of things, but I'd hesitate with a floor job.
General Finishes does make a couple of very intensely dark oil stains, like Merlot (reddish) and Java (brown). They are capable of giving as dark a stain as anyone could wish for. They are also quite grain-obscuring.
Casey

Check out Bona Kemi. They make a finish for professional floor finishers. You can call the guys that sell it on line, tell them your problem, and they will tell you what to do. The method for applying the product is simple. The applicator is 18 inches wide and you use it like a snow plow or at an angle.

Thank you for the input. I can tell there is knowledge and experience here. For the record, I don't believe we would be putting a dye over an oil-based stain; I was simply guessing at the process. Our wood is bare now, so we can take any direction at this point. I am definitely not okay with odd fading patterns, if that is the case with aniline dye. I will look into Bona products. Thanks for the advice!

"I am definitely not okay with odd fading patterns, if that is the case with aniline dye. "

Some colors hold up well, while others are fugitive (fade in light).

The browns are usually stable, while the reds are very fugitive.