The starting point is to find out what the 23% relates to?
You are right in terms of EU VAT treatment before BREXIT ie:
UK to EU (dispatch) – zero rated of business & VAT registered customer otherwise charge UK VAT.
EU to UK (acquisition) – reverse charge VAT
Business (A) to Business (B) - VAT place of business is country B.; therefore, services from UK to EU to business EU VAT applies to the business and hence zero rated in UK, quote VAT number and include on Sales list. Business to consumer – place of business is the business hence UK to EU, UK VAT applies.
However there are cases where one may need to register for VAT in different EU countries if your client supplies cross-border telecommunication, television, and radio broadcasting, or digital services to non-taxable persons. You can google more about this on or find out here: https://europa.eu/youreurope/business/taxation/vat/vat-digital-services-moss-scheme/index_en.htm.
Otherwise, I suggest you get more information from your client.