Sil wants wood over existing wood.
She sent me a pic of some 3/4 thick stuff, is it common to do this? It's going to be a pretty good jump from kitchen vinyl to the wood.

You see lots of wood over wood on the internet.

I wouldn't do it.  I wouldn't want that kind of offset.

It’s been done, my ocd won’t let me, but it’s been done.

she just needs to be 100% on what the current thickness of wood it is, how thick the subfloor is. if its 3/4" thick over solid 3/4", shell have to run the new floor perpendicular to the current, and will likely have to cut all doors and door jambs, and will have to use a reducer which is going to feel like a ramp. she should just remove the old floor so she doesn't have to dick around height issues. you don't want to run it parallel or else you run the risk of the fasteners going through the tongue/groove portion of the old flooring, which wouldnt hold entire rows at a time. the old floor wouldnt be in serviceable condition if you ever remove the new floor, since it would remove chunks of the old floor and you cannot hide the staple/cleat holes, so removing the old hardwood probably ends up costing a little more than all the extra modifications eventually needed to compensate for the additional height. becomes unsightly if there are any steps, or non-hardwood areas on the same floor, and could run into issues for sliding doors or metal front doors.

Remove the baseboards and old floor. Replace with new floor and install new baseboards.

Do it right.

Outside of what has already been said (the negatives) there are some positives to it. Basically the old wood acts as a second layer of subfloor which can reduce noise transmission to lower levels, reduce thermal transfer, stiffen the floor, etc.

If the installation is properly planned to mitigate the issues outlined above AND she really doesn't mind the large step up I don't see any problems with it so go for it.

If the current wood floor is at or near existing subfloor level of adjacent floors many of the problems will be non-issues but that would be a rare install although I have seen it in newer homes that were designed around having wood floor meeting adjacent floors that weren't wood to keep them level).

I wouldn't do it in my house. Taking out the old floor is so quick and easy that it isn't a factor.

Thanks for the replies,  it'll be her decision.
The base boards would be coming off irregardless of which flooring goes down. I'll only have about a week for the job, plus paint the outside (two story) and a couple of rooms inside.
Some vacation ??

Depending on the existing wood floor,  it might just be best to refinish it.

I hope you have several helpers if you only have a week to do all that. Either that, or you are superman.

She's leaning towards refinishing the existing floor now.
Is there anyway to deal with the holes left in it when I pulled up the carpet a few years ago?

Holes from what? Tack strips? Wood floor filler

Back from workcation.
Ended up sanding the floors, stain and clear poly over. Only poly'ed one br and dining area, as its a house full of f*ckin junk that had to be shuffled around.
Painted a br first, it needed primed due to her grandkids writing all over the walls, so moved stuff in there back and forth 4 times.
Going back to finish poly, painting etc.
At least she pays well.