Tomatoes are tricky to substitute.

The first thing that pops into my head is slices of plum. However, this strikes me even as I write it as a little bizarre, and I should warn you that I am still recovering from Hogmanay lack of sleep and may be making no sense. But, well, they are sort of juicy and sweet-sharp, aren't they? And red...

I don't know.

Mexican BLT...... I'd say avocado and/or cucumber.

I watched a show on the food network the other day and the restaurant used a tomato jam, instead of the sliced tomatoes.  It looked really delish.

When you say your friend doesn't eat tomatoes, is this an allergy, or just a preference? If it's a preference, is it for all tomatoes everywhere in every context, or specifically for raw tomatoes or tomatoes in tomato shape? e.g. my Mum hates raw tomatoes or recognisable cooked tomatoes, but is fine with tomato sauces and chopped tinned tomatoes in hot dishes.

Just thinking you could add tomato flavour with a bit of tomato puree or similar, but if it's an allergy you'll have to avoid them altogether.

Missing them out is the simplest option.

Thanks for all the good advice guys! I decided to just sack it and throw the tomatoes in anyway - I didn't actually realise they were in the guacamole!

However, Ashen, you did make me go out and get some prosciutto to have melon / prosciutto tonight!