In order to be prepared, have in your own mind what you are looking for and what you would be comfortable paying. Also, understanding where the seller is coming from will help you -- Does he need money? Is he ready to just clear things out and wants her items to be used? Is he actually ready to part with her things?
In a situation like this it is helpful to find out early in your shopping/negotiating process what price the seller has in mind. It is very awkward to be in someone's home looking at things for sale and find out either that the price is much higher than you had in mind and/or the quality or selection of the items is not what you want. I usually ask at the beginning how they are selling the fabric -- by the yard, bolt, box, etc. and than ask them to give you an idea of approximately how much they want per yard, etc. If it is within reason to you, then you can start making selections and settle on specific pricing as you go.
If you find there are a large number of items that you want, then try to agree on a "lot" price for all of it.
Above all, all keep forefront in your mind that this is a sensitive situation where a man is selling things that were dear to someone he loved. You will have many opportunities to acknowledge to him your understanding of his feelings and your appreciation of him allowing you to be there going through the things and purchasing them. You can easily make positive statements about the woman's taste, quality of materials, colors, etc.
This is an opportunity to be a win/win situation. You are in a position to help this man in his grief journey.
Dayle