Disclaimer: Do I own any of this other then the idea. That would be a NO
The rules of society is it's polite to offer and it's polite to gracefully decline. Lucy however in her distress forgot all about curiosities. The life of her husband was at stake and Sydney did say, "for you and those dear to you I would give my life to keep alive a life you love" So when she saw Sydney she couldn't help it.

"Sydney you made me a promise once"

"I did?"

"Yes. You said you would do anything for me"

Sydney nodded

"And I meant it"

"I'm going to sound like the worst friend in the world, but... I need to ask you for something"

"Of course"

"Did you mean it when you said you would give your life to keep alive a life I love"

"You know I did"

"Would you... trade place with Charles tomorrow."

Sydney liked at her wide eyed

"I had planned on it," he told her, "but this time I must charge you. The price is one kiss goodbye"

And with that he planted a gentle kiss on her lips and walked away for the last time. As he looked back he called

"Until we meet again in heaven"