I voted Yes, but she should say nothing specific about their relationship, if possible.
A funeral is not the place to confront anyone, out anyone or behave in any way that might add to the grief of close family or friends. That's just cruel, regardless of what the grieving family did, or did not.
I was in a somewhat similar situation at my father's funeral. My parents had been divorced for long years, yet had become friends over the last years of my father's life. But they also still had big differences, there was a reason why they were divorced.
Imagine my surprise when, at my fathers funeral, the sermonizer (not sure about the word, he was no clergy) announced to the whole community that my parents had planned to marry again! I knew for sure that this was a plain and simple lie on my mother's side.
I let it slide and was happy nobody asked me for details. I just let them believe whatever they did. A funeral is not the place for such things. (Much) later, when family members asked me if that was true in a one on one talk, I told them it wasn't true, but let be, let be. So through the grapevine the family knows what's what, and who does it hurt when some old collegues, neighbours and other acquaintances believe they were about to marry?
I know the situation is not the same as in the Prudence letter, but the principal is the same. And you do not disturb a funeral, period.