Hi Sebastien,
It's very easy (as you mentioned already :-)).
Click on the tab "Users" in your CAL management console and click on "HELP" on the right-hand side.. this displays context-sensitive help and you will find all the relevant info. on managing users in CAL.
Step-1:
Ask your colleague to send the access URL for his CAL account to you via e-mail. Your colleague can copy it from the browser and it should look something link this:
https://cal.sap.com/console/tenant_A7V9xxxxxx
Step-2:
Click on the URL and click on the link: "apply for request"
https://cal.sap.com/console/doc_en/frameset.htm?46948f72469c42c88e1735fcb7aea529.html
Step-3:
Now, your colleague will see your user in the list of pending users in his account (under the tab "Users") and he/she needs to "approve" your user and "set the role as Administrator".
https://cal.sap.com/console/doc_en/frameset.htm?efcefd53d0284d0bb5bec4ec02d42085.html
and that's it, you're done!!
You can now access his CAL account and all the solution instances by using the same URL as mentioned in Step-1.
Best Regards,
Adarsh