There are 2 cake recipes I can recommend. On the recipe thread on here you will find StaceyUKs lemon cake which is lovely (I think its 14 syns for the entire cake and on www.slimmingeats.com (a blog from SW member Britmumincanada) you will find a squidgy chocolate cake- I think it's 9 syns for entire cake.
Other than that I think a seafood platter would be an amazing birthday treat- a bit expensive but lots of smoked salmon, prawns, mussels, crabsticks would be a real treat
Also, the Spanish orange cake from the SW website is excellent. Very tasty.
You could buy (or bake) individual cupcakes for you and your friend, that way you might be less tempted to eat more than one. Once I've made a cake I tend to want the whole thing!
The buffet foods I mentioned above should all be good, and SW friendly. In addition you could do some cold meats, boiled eggs, corn on the cob halves...
There is lots of info on the SW website and if you search this forum you'll find quite a lot of ideas too.