Diversication is defnitely important. Google is too unpredictable to have it as your only money source. I do believe that properly done PBN Networks won't get hit, none we have made that was done carefully ever has, that said, you never know what Google might come up with next time...
Out of interest, when you said your client's PBN had too much volume, what did you mean with that exactly? Too many outgoing links?