Thank you all for your suggestions. Yes, the problem is that she has some difficulty equalizing especially at the beginning, and she cannot just dump the air, for then she would be descending too fast.
This seems to be a more general buoyancy control issue. There is no reason why a descent should be fast in a horizontal position. If anything, being horizontal would increase water resistance creating a slower, more controlled descent.
A preference for vertical descents tends to indicate that the diver places a high reliance on using their fins to creat upwards thrust to compensate for an underlying buoyancy control issue. i.e. the diver is not achieving neutral buoyancy and using fins to counteract their negative buoyancy.
Address this issue by first ensuring that her weighting is appropriate. Over-weighting often causes this type of problem with descents.
Secondly, work to develop more precise buoyancy control, thus eliminating the necessity to rely on the fins to arrest/control the descent.
Also, she feels that being horizontal somehow makes the process more difficult and less comfortable.
As mentioned, being horizontal relies on effective buoyancy. Without that buoyancy control, the diver is likely to find themselves struggling to achieve a comfortable descent. It truth, a horizontal descent should be painless and stress-free.... nothing more than a gentle drift downwards, which can be arrested with breath control.
With regards to drift diving, being in a vertical postion will only maximize the impact of water movement on the diver. Unless the divers' intention is to move with the current at the fastest possible rate (
divers' would rarely have that goal), then horizontal positioning is by far more preferable as a drift diving technique.
In addition, her feet are light possibly in part due to the neoprene socks she is using, and sometimes she ends up with her feet pulling up. We will try to lower her weights a little, and work on her buoyancy during descent so she can remain horizontal more comfortably.
In addition to what others have contributed, I would recommend that your wife also investigated the relative buoyancy of her fins. It seems likely that she might have very 'floaty' fins. There are several models of fins available on the market that provide substantial negative buoyancy... and these would effectively counteract the lift provided by her booties.
Body positioning makes a difference when attaining good horizontal trim. Extending the arms forwards in a relaxed pose can shift some gravity forwards. Likewise, holding the legs upwards (bent at the knee) will reduce the 'lever' effect of having postive buoyancy in the legs. In addition, a weak posture can make horizontal trim very unstable and difficult to maintain comfortably. Creating a small arch in the back, with the mid-torso forwards and the shoulders/knees raised will make the position much more efficient (
a bit like a skydivers' pose). This can take some practice though.
Overall, I would suggest some consideration of potential underlying problems; especially weighting, buoyancy control and trim. Without these as a foundation, no amount of trim weighting and tank shifting is going to have definitively beneficial effect.