In reply to Timmd:
Custard is easy to make. Use corn flour and a non milk alternative (coconut or hazelnut would be my preference.) A splash of valilla essence, some sugar and that's pretty much it. If you wanted it yellow, then a drop of colouring would do it. If you're worried about it not being eggy, then simply cal it coconut / hazelnut custard. If it's too thin, add more corn flour and bring to the boil.
I appreciate that your friend is not vegan, but there may be some reading this that are, jelly is not vegan. Your local friendly Indian supermarket will have jelly, which is made with agar instead of dead cows.
Does your trifle have sponge in it? I guess you've already sorted out a vegan sponge recipe.