Non-spicy substitute for chili peppers when making a Thai curry paste from scratch
Let me take a wild swing - 

I think trying to generate the volume required by using jalapeños would be too hot for your friend &amp; you're right that a regular capsicum/bell pepper wouldn't have enough flavour to water ratio. Also 'bell peppers' don't taste like chillies.

How about Hungarian or Banana peppers?  

Now, this is quite a variable &amp; may depend on where you live as to which type you can access. Here in North London I can choose from 3 varieties without going more than 50m from my front door - from the the fat 'triangular' ones like giant fresnos with very smooth skin which are very cool, to the skinny ones about 20cm long with a 'lumpy' skin, which would be the type I would aim for.
Though they are generically known as Hungarian, [&amp; of course, could actually be grown anywhere] they are likely to be commonly found in Turkish stores - local availability of course not guaranteed.  

They taste like 'chilli' not like 'bell peppers' but they really don't have too much kick to them. They would generate the flavour without too much heat, but not sufficient colour, so you could perhaps trick some extra coriander [cilantro] into your paste to deepen the green.

From comments - though I wouldn't bother for myself, you can keep the heat down still further by not using the seeds or inner 'whiter' membranes.