It seems to me that he could upload the attachments to his post, left click on the generated post so they open in lightbox, then right click the image and choose "Copy Image Location" (or however it's worded in his browser), then copy that URL into the [img] tags; I have done this for years (well, minus the Lightbox step) on my PC and Gardening communities, and it's pretty simple to do.
I don't see what could be done to make it easier on him other than to rewrite the system, which could at some point happen, but there are so many items that need attention short-term during our race of getting things updated to display as modern web components...
