Would she be willing to consider hypnotherapy? He holding onto the items can be triggered by her anxiety. If she did not respond well to the counselor who had confronted her, hypnotherapy offers another way for her to change her behavior.
Her fear of forgetting will have to be dealt with. She may be able to photograph some of the things ex son's projects, etc. and put the information onto a CD.
You won't be insensitive helping her find some solutions for the clutter. She may not see it from your stand point and you won't be forcing her to adopt your view. Find out what she is willing to do. If you present her with some options, she may adopt one of the solutions.