This is the best method I have read, so far:
https://www.reddit.com/r/TheSilphRoad/comments/8ugiow/you_can_synch_an_out_of_synch_friendship_so_that/
We're definitely stuck with Niantic...
Edit. I also found it visually explained in this other link:
https://www.reddit.com/r/TheSilphRoad/comments/8uj11a/visual_illustation_of_how_many_days_you_can_save/