Hi there and welcome
Please do not shoot the messenger but it is not possible to unsend a message, once it has reached there phone, you have no way to access it and remove it. It is the same as trying to un dend an actual letter. Once it is in their locked mailbox, it is not possible to obtain and remove it again.
Again, sorry to have to deliver the bad news but hopefully it saves you some time in the long run and prevents anything like this from ever happening again.
It would be greatly appreciated if you could rate my reply to you by clicking 3 stars are more. I of course understand this is not good news but you are just rating me personally and not how messaging works and their limitations
If you need anything else, please let me know
And please remember the service rating via the stars at the top then submit to close this off.