Thank you and yes floral I did tell him the pit should have been visible when he dug it. He said he had planted an entire pit as well as one that he had cracked and removed the seed from. I feed this fellow only as much info as he can absorb at one time and he is improving. I knew it certainly looked unlike my wild plums.

I'm guessing it's a Glossy Buckthorn.

Oh dear, lol! Thank you Dave. He would be disappointed but he would eradicate this invasive. I see a lot of common buckthorn and would have recognized it but the glossy has perhaps not been on my radar.

Bump. Hoping for a consensus on Glossy Buckthorn before I tell this fellow to rid himself of it.

Frangula alnus, Glossy Buckthorn

Thank you Jay for confirming what Dave had thought. Pity but it needs to go. Hopefully the fellow misjudged where he put the plum seed and pit.