Good morning! I am not on South Beach so I can't comment on making the recipe more friendly, but I understand not wanting to derail process.
If you decide to make the cake you can always only allow a reasonable piece on birthday and then throw the rest out. I know that seems extreme, but it is an option.
Hopefully there will be a good cake option that you can enjoy through the week and not worry about being off the diet instead.
Happy Birthday Early! Your husband sounds like a good guy.