I don't know what it is, but i cut one down on my property. It grew from seed and within two years, it was about 25 feet tall! I have seen them planted in parking lots here and also growing wild (not sure if native or introduced) in the foothills of the Sierra Nevada (Millerton Lake to be more specific).

I haven't seen any glossy buckthorn here, although that doesn't mean it's not around! Google is a mixed blessing, because I checked the buckthorn images and decided those leaves have more prominent veining and longer, more distinct stems, but then followed up with the M. virginiana images and ... they were no more similar. Bill's point about leaf arrangement is interesting; I checked wikipedia on this and both plants have alternate leaves. That might be more pronounced in the buckthorn, I don't know. Wikipedia did say that Rhamnus has brown shoots, and this plant's shoots are definitely green. LOL, I guess I should just inspect my other M.v.s and see if they have the same insect damage (leaf notching) as this baby - looks can be deceiving, but bugs are never fooled.

I thought it looked like a UK house and garden but was fooled by the OP name ;-) No way of knowing what sort of cherry it came from but since they don't come true from seed, even if it is from an eating cherry, it won't produce the same kind of fruit. It could also be from a wild cherry, Prunus avium, brought in by a bird. All your friend can do is to wait and see what the fruit are like.

It looks like a young plant . Kentucky Coffee Tree has pinate leaves and your plant doesn't, but I don't know the morphology of young plants. The leaves of the true species are confusing because they are slightly alternate, but some pair up and look opposite.