invasive plant id

hi this is my first time on here and its a question about a plant which is growing in my neighbors garden. she is an old lady who cannot keep up with it . it grows along under her lawn and pops up every so often she has told me it has gone through her greenhouse floor and has strong root i have added some pictures and hope someone can come up with a solution please .... kevin

 

Orchard plum.

 

i dont think it is a tree as such as it seems to travel along like a stinging nettle

 

RON B ....you are right there is a plum tree down the bottom of her garden ....leaves match up .... anything she can do about it ? please