Which mourning character does the second narrator visit Europe at end of 1902 novella?

The second narrator in "The Ambassadors" is Lambert Strether, who travels to Europe at the end of 1902 to "rescue" his young friend Chad Newsome from the influence of the alluring Madame de Vionnet.

So, the answer to your question is Madame de Vionnet.

It's important to note that "The Ambassadors" isn't a novella; it's a full-length novel.