No, he would only have to add things (Clients, Projects, etc.) that he would work on. I cannot remember if he adds it, since it would have a new "modification" date, if it overwrites your version on sync. e.g. if he adds a Client with only the name, it would overwrite your Client record with that info, or it leaves it alone. I do not remember the one-way sync rules. If you can try it with just one record, that would be the way to go. Do a Backup before you try it, so you could Restore worst case.