Character missing

A friend of mine hasnt played since the WoD/legion era, I vividly remember grinding toons with him and ranked pvp. In recent days he has tried logging in and his account that ive had added since 2017 has none of the characters he has worked on, as well as missing more the 70% of his mounts, gold, and achievements. This has deferred him from continuing to play retail. He has a random assortment of toons that he may have made but had faction changed to alliance. Im thinking its a server merge issue and its backloaded an older copy of his account. It doesnt seem hacked or characcters deleted. And we’ve tried to do the new character on a server and clearing the cache in task manager, but it doesnt help. Any advice would be much appreciated.