I have never grown the species, but I think Prunus sargentii is known to behave this way.

I have Prunus virginiana ’Sucker Punch, and the leaves emerge green, maturing to dark purply-red.

Thank you both! That P. virginiana could very well be it. I don’t think there is any Japanese cherry in the neighbourhood, and the mature leaves are clearly purplish. I will tell my neighbour it is a Prunus, then she can decide what to do with it.

Prunus virginiana 'Canada Red' is commonly grown as a street/landscape tree around here and looks very similar to yours. I believe 'Sucker Punch' was a later development that cut down on the prolific suckering this species is known for.

This looks like being a sucker from under the fence, so quite possible that it is ’Canada Red’. And very appropriate here in Canada ;-).