Look like unripe Loquats, Eriobotrya japonica. It's not a tropical, grows in cooler climates.

Pretty sure that's a Walnut tree with green walnuts. As far as I can see the leaves are pinnate. Certainly they don't look like Loquat leaves. Does Loquat even survive in z6? Green Walnuts

I didn't notice that the leaves were pinnate. That does take Loquat out of the equation. But otherwise, the similarity is amazing. The tree isn't growing at rredbbeards place, it's in San Francisco, don't know the zone but I think is a bit warmer than zone 6.

The location of the mystery tree is zone 8a or 8b, so loquat would be ok there, although this isn't loquat. I'm going to ask him for another picture of the tree.

--Rr

Sorry - didn't spot the San Francisco mention. Just looked at the OP's location. But my id is the same. Did you look at pictures of green walnuts? That's what it is. Green Walnuts on tree. If your friend cuts one open he'll find a pale embryonic walnut inside. If he has a lot he can make pickled walnuts.