There's several issues involved there. For starters a "human rated" rocket needs to operate within certain performance envelopes and have certain features. For

There's several issues involved there. For starters a "human rated" rocket needs to operate within certain performance envelopes and have certain features. For instance the rocket carrying a human crew will have different vibration needs than one carrying cargo. The rocket also shouldn't vent cryogenic gases to the atmosphere while on the launch pad which creates a danger for astronauts and a ground crew when the rocket is fueled. The Ariane hasn't ever really gone through a human rating process and doing so would take several years. Even then, the ESA does not have a crew launch vehicle available. The Automated Launch Vehicle was used to resupply the ISS but can't carry a crew despite having a habitable volume when it was docked to the station. The ATV doesn't land but burns up in the atmosphere at the end of its mission. That's not a vehicle astronauts would really want to ride home in. While I don't doubt the ESA could develop a manned vehicle, it would just end up being so different from the existing ATV that it would take longer to develop than the remaining life of the ISS.