Hmm, first you are not the only person whom has dedicate their novel to someone living or has passed on, so I hope you do not feel that there's any issues with doing so.
Some authors' have stated this help to with the loss of that person and others have created a character in their memory into their story.
I rather like the idea, seeing letters at the end but if at some point you do find it getting hard to write them, there's no reason to try forcing the matter either as it could mean you are at peace with their memory.
Cheers