It's still pretty fresh for her. Both of my parents have died and it is heartbreaking, I could only imagine how difficult it is for her to lose a child.
Maybe the retreat will be good for her, to grieve with other parents who have lost. (Managing the loss of a loved one is a process.)
You may want to have an 'intervention' with her of sorts and let her know that she has other children that need her and share in her grief, as well as any grandchildren who need their Grandma...... Just an opinion, but she needs to feel how much the
living around
her need her here, her son is with Jesus (or the equivalent) and she will be with him someday again, but today and tomorrow, you need 'your Mom' too.
Good luck, this is probably so hard for her (and you).