Fred had sat through the whole proceedings, with Katie at his side, dressed in her Auror robes to pay respect to their new brother-in-law's Father. He could tell Neville was pained and conflicted, even while he said some wonderful things. He didn't know for sure what his father had been like and that's what made everything so powerful.
Everyone had been so ecstatic that Ginny and Neville had run off to get married - having another brother was more than welcome, and he made Ginny so happy, they were all glad to have Neville in the family now. And Ginny stuck to him like glue, offering support to her new husband, as he obviously needed it. Fred couldn't imagine what Neville was feeling - knowing but not really having that parental figure, well figures, around his whole life. He felt very bad for Neville, but would be there, right along with the other brothers to cheer him up and make him feel a little better.
He and Katie stood after Frank had been laid to rest and wandered around until they saw Ginny and Neville talking with Harry. "We should go and give our condolences after Harry."