Plum aphids do sound most likely but if you didn't see any then that's probably not the culprit. No honeydew on surrounding leaves?
Did you happen to notice any white spots on the fruit? if so, then it could be a disease called plum pocket. Could it just be environmental with all the cool, damp weather?

I have heard of Plum Pocket and the first thing that entered my mind was aphids for sure. However aphids usually make the foliage curled and crinkled looking. I noticed some small green flies on some of those same leaves. I believe Green flies may cause the same problems as aphids. It's funny that only one branch on that healthy looking tree has the problem. I'm thinking she should cut the branch off, what do you think? Possibly this one curled up branch is just the start of it and the entire tree will be infected soon? Cheers, Joe

bananajoe wrote,

"...small green flies on some of those same leaves."

Those could be winged aphids. They come in various colors.

Often, by the time the gardener notices the curled/distorted/crinkled leaves, the critters have completed their job and are either dead or gone. So you may see live aphids, their cast skins, or just grungy debris.

With plum pockets, the leaves are distorted and *swollen* -- quite different than aphid damage.

My plum gets one or two small new growth branches each year that have smaller leaves, curled a little and are closer together.
The twigs on these branches are also closer together and a little twisted looking.
It kind of looks like a witches broom effect.
I have just usually cut them off.
There seems to be a little more of this happening on the tree this year.
I was wondering about something like peachleaf curl.

My tree is generally a bit stressed and old, though.

I'm also thinking that possibly a small boring insect has made it's way into that branch. The entire tree is very healthy and loaded with fruit. It's just that branch which has curled in foliage. I know what aphid damage looks like. The leaves are often curled in and crinkly in appearance. I did see one or two green winging aphids, but otherwise that branch was pest free. I know it's not Plum Pox either, that is a rare disease around this region. I should tell my friend to remove that one branch. I think she's worried it may be a disease or the start of one. Thanks for all of your advise. Cheers, Joe