Best friends move away. Distance tests which friendships were proximity and which were real.

Plenty of closeness is manufactured by seeing somebody constantly, which is not fake and is also not the same as choosing each other. Distance strips out the automatic part, and what remains is whatever both people will maintain on purpose. Some friendships end right there, and that does not turn the earlier years into nothing.

If somebody has moved, be the one who builds the structure. A monthly call, a show you both watch, a visit with an actual date on it. Maintenance is unromantic and it is the only thing that works across distance.