We operate in the reverse to your friends situation, Irish Company, over 50% of our business in the UK. I looked into this a bit and my understanding is that in our case we are an Irish Company, based in Ireland so pay taxes, Vat, etc here. We charge Vat of 0% to the UK customers, noting on invoices that it is subject to reverse charge. We have to make VIES declarations noting all trade and Vat numbers of UK clients. All sales are included on VAT3 forms under sales to other EU countries.
There is often a distance selling limit that once you go over you need to register for Vat in that country. Below is the case in the UK .But note that if their Irish customers are registered for Vat, then it is not deemed distance selling and according to HMRC :"Distance selling only occurs when a customer isn't VAT-registered."
On the HMRC site there is an explanation about what is and is not distance selling, link here, [broken link removed],
According to this selling from one EU country to another EU country is not distance selling as long as your customers are Vat registered. Maybe someone can clarify if this is the same throughout the EU or if Ireland has a different set of rules as to when another EU country selling to Ireland needs to register for Vat in Ireland. Hope this helps.