That depends on the country.

Please check reopen.europa.eu for COVID requirements. Do take into account that the rules change often and with little notice. What was valid last week may not be valid now, let alone next week or next month.

It is not necess, perse, but it will in SOME countries bring BIG difficulties still-as a rough rule-the further south in EUR (IT, ES) the more hassle. Also friend will probably have to get tested much more often as expected-at own expense. Vaccin is only valid after 14 days in most cases.

It is still the case that in some countries you can only use HTL/restrt with that QR-code that you only get with vaccine or recent test. This will bring more of these problems as trainriding itself