Not unusual. When potatoes bloom sometimes they set fruits. Since they are close kin to tomatoes, the fruits do resemble cherry tomatoes. Modern cultivars are developed for maximum production of tubers, so they usually bloom and set fruits only under conducive conditions.

So let me get this straight ;o)...their potatoes are happier than ours because they've set fruit and ours haven't? Gives me something to strive for next year LOL.
Thank you for the feedback and answer to the "burning question of the evening".

Do not eat them - they are poisonous.

poisonous ? I hope you didn't post that too late

rose ~ Poisonous? The small fruit or the potatoes? I'm hoping it's the fruit/growth you're referring to. Our friends don't plan on eating the tomato-like fruit, just the potatoes. I know potatoes and tomatoes are in the nightshade family, so now I'm concerned as to what I should tell them.

...when I "Googled" a bit further:

"Occasionally gardeners are surprised to find small, round, green, tomato-like fruit on their potato plants. These fruit are not the result of cross-pollination with tomatoes. They are the true fruit of the potato plant. The edible tubers are actually enlarged, underground stems. Normally, most potato flowers dry up and fall off the plants without setting fruit. A few flowers do produce fruit. The variety 'Yukon Gold' produces fruit more heavily than most varieties.

The potato fruit are of no value to the gardener. Potato fruit, as well as the plant itself, contain relatively large amounts of solanine. Solanine is a poisonous alkaloid. The small fruit should not be eaten. Since potatoes don't come true from seed, no effort should be made to save the seed."