My partner has arthritis in her hands such that she has trouble gripping the paddle with her thumbs. Rather than a normal grip with thumbs under and fingers above, she must keep her thumbs above/next to her fingers - as if her hands were in a sock or a mitten with no thumbs. Are there any adaptive devices or gloves out there? Thanks!

Yes there are
There are devices that essentially do the gripping for you by wrapping a strap around the paddle shaft. Try a google search for “adaptive paddling devices.”

I’ve also heard of people creating their own such devices.

In calm water I often use the thumb-on-top grip to relieve pressure on the thumb joint.

Does she already have a small diameter
shaft paddle? Investigating bent shaft paddles with low feather angle might also be worthwhile. It would provide wrist angles that might make what grip she does have more effective. The trade off is that bent shaft paddles are a little heavier than straight shaft.

straight shaft here, but going to a smaller diameter shaft helped my wrists.

Some examples

Also google “adaptive paddling device.” I’ve seen a better device than the ones above but I can’t find it now.

good links
Following this I found a glove with more stickiness and texture to help those who can’t grip the paddle shaft well enough.

1st MP joint arthritis
is very common over 50 and a pain for kayaking. If your friend is willing to try it, a local non-steroidal like voltaren gel can be quite helpful. Ditto for the occasional cortisone shot if she’s having trouble opening jars etc…

Go get a pool noodle or insulation for piping (cheaper) you can put the insulation around the paddle to make the diameter larger, making gripping easier - secure with electrical tape.

Probably too big
When the paddle diameter is too big it causes stress on the base of the thumb. You can wrap one layer of thin neoprene around the shaft and secure it with tape.

arthritic paddle grip
Sorry for such a long delay - Thanks to all for your suggestions. I’ll pass them on to her. I didn’t know about the strap. We use narrow shaft paddles, so doing the noodle wrap would be a good test for a larger grip. BTW - noodles are also great as knee pads. I cut 2 12-inch lengths, slice halfway thru lengthwise (noodle is hollow-cored) and pop them on the cockpit edges.