When will Fort Aspenwood not be full?
It may be months, it may be hours. Server population is determined by WvW activity now.
The only guaranteed way to play WvW with your friend is to transfer to non-Full server. If you don’t care about playing WvW with him/her, then it doesn’t matter which server she/he chooses, as long as they are in the same region.
Good luck.
It’s based on WvW activity during peak hours too, so waiting around for people to log off isn’t going to help either. Basically, the only way your friend will get onto Fort Aspenwood is if there is a downward population trend in WvW that causes it to get downgraded from Full or they increase the population cap.
As IC said, your best option is for BOTH of you to play on a less full server (or skip WvW entirely and then the server doesn’t matter).