I understand, only too well, how special these little dogs become when they are ill and you feel that they will not be with you for long, but Karlin is right, you may be just taking on more heartbreak.
Unless this murmur was very sudden onset, I would really question the wisdom of obtaining any puppy from a breeder that would use a dog with severe MVD. What other health problems may she be ignoring in her breeding cavaliers?
Please do not reward this breeder for what sounds like very irresponsible breeding. If you have room for another puppy, buy from someone that does all the health testing, they are the breeders that should be supported.
Cavaliers......Faith, The Ginger Tank and Woody.
Japanese Chins.... Dandy, Benny, Bridgette and Hana.
Remembered with love......... Tommy Tuppence and Fonzi