Hi Nancy,
That's funny about your mother dreaming that I explained to her what this dream meant last night because I did read this thread yesterday but wanted to mull it over for surety. When I believed that I knew what this dream indicated, I didn't have the chance to return to the forum because my family went out last night.
The coffin as a dream symbol can represent many things. That can be said of every dream symbol which is why some dream psychoanalysts like Carl Jung dislike dream dictionaries. You just can't have a single meaning for one dream symbol. Context and the dreamer's life history both influence the symbolism.
But with that said and going on the surface description of your mother's dream, it appears that she is preparing to bury some issues in her life. Perhaps there are parts of herself, her past or even her part in whatever life challenges she or her family is facing. And she would like to put them to rest.
And she would like some help in doing so. If there is an opportunity to forgive and forget, please help her by not dredging up old wounds or past infractions she may have committed against you. It is time to bury old resentments and hurts.
Hope this "matches."

If not, I would love to hear what she dreamt I told her.