@ Ruben. Your post is unrelated to Morgan's comment.
@ Morgan, ask your user to click on the up or down arrows to the right side of the post editor buttons, to expand the edit box. that uses universal JavaScript and works fine on Edge and all other browses without needed any additional software or file updates.
The up/down buttons are located on the far-right of the "Full Reply Screen" -- They are in the top row.
I've just tested them on ubbcentral using MS Edge. That should get your MSEdge users back up and running again
Cheers