Omg you had the same problem I had. It's really tricky. Friendship event 2 or 6 I think... Well when you go through the portal and you need to collect cubes, exit as soon as you can and there should be a event for her. Her events are at the house or at school. Always go to school!!! Get all her events. Don't worry about the portal, it will stay open and you can always return to it after seeing all her events. You can also catch all of madoka's events that way too since they are a bit out of the way/hidden like mirai's.

Turns out it's event 3. Well don't forget to do home and school events. Since she is 99% of the time there except that one time when she was outside in the city. Make sure to leave right after seeing the first cube during the dialogue and do not return until finishing all the events.