I don't know about her case being terribly strong, although I do applaud the sentiment behind it. She will have to prove that 1) Sicknick's death was a direct result of the attack and wouldn't have otherwise occurred, 2) Trump was clearly culpable in causing the attack, and 3) Sicknick was supporting her, his death caused financial and mental hardship, and that he would have continued to support her had he lived. This part would have been stronger if they had been married. We don't know at this point what their relationship was.
I hope that it doesn't get dismissed out of hand for lack of standing or something. Good for her.
1. Sicknick's death was listed as in the line of duty.
2. The standard is preponderance of evidence, not proof beyond a reasonable doubt.
3. Wrongful death lawsuits don't require proof of financial hardship otherwise we would be living in "The Purge of Rich people."