There is enough idea in here and I don't think I can add anymore.
From what I see, what she needs is a good friend to help her out and getting start. I think she has found the good friend --Pete!

: I have a friend who would like very much to try kayaking. She is young and in
: good physical condition, but has one problem that has me asking for help.

: Born with a birth defect, she has the first joint of the right thumb, and the
: first joint of the right pinkie, but no other fingers on the right hand.
: The palm is otherwise normal, and there is no atrophy of the arm muscles.
: She is strong enough to paddle, but gripping the paddle is the issue. The
: digits are not long enough to touch in opposition. Furthermore, she
: doesn't want to invest in a paddle unless she can actually use it, so
: permanent modifications are problematic.

: I was thinking about a pogie or something like it, or a glove with velcro on
: the palm and temporarily attached to the paddle. Any other ideas?