I am interested in your question here. My understanding was that you only have to provide an invoice if both you and the customer are VAT registered - if you are not VAT registered a sales receipt is sufficient documentation.
The following implies the above
If the customers are businesses in Europe then reverse charge VAT may apply - and an invoice is required. I.e. no VAT is charged on the supplier invoice but they must state that the sale is subject to reverse charge - and the business customer must account for the reverse charge in their own country. It is easy to get one wrong so your client needs to check on this.
Services supplied to countries outside europe are generally outside the scope of VAT and therefore I assume a sales receipt is sufficient documentation.
I would be interested to know whether this is incorrect.
Edited at 29 Jul 2020 03:05 PM GMT