Cannot rejoin party until it reaches a lower depth?
So here's the situation: I was helping a guildie get to the JK levels so he could go solo and get some practice fighting jellies/kiting without other annoying party members in the way. I left him at depth 15, went on to finish the rest of the JK levels, and headed back to Haven. A few minutes later, he PMs me asking me to rejoin and revive him -- he went down towards the end of depth 15 and didn't want to use a 20 energy revive. However, I couldn't rejoin, saying that his party was "no longer accepting new members," and he couldn't invite me, saying I "couldn't rejoin until the party reached depth 16." Anybody know anything about this?
Yeah that was my next question.
Is this restriction always present? If it isn't I'll just tell him to go down a different track or something.

Yeah, it's always present, I've had it happen on just random clockworks depths.
This seems to be a measure put in place to prevent you from exploiting "join a friend" and "go solo" to loop through profitable levels repeatedly.