I've found CryFS doesn't like multi Gb vaults, gocryptfs is happy regardless.
The larger Vault is open but Dolphin acts weird about it. You have to use the terminal to copy out the files from the mounted Vault and just wait for it to complete unfortunately.
You probably want to check your wife's Onedrive online with a web browser as by default Windows doesn't keep files locally in Documents/Desktop/Pictures etc.