Two things we should take into account:
1) "all" there refers to "needs," not to "clients," right?
2) "client" usually refers to a specific client but I think it can also be used generically, meaning "the typical client." In that case, the singular would be acceptable, but I agree with you that the plural would be correct too.
I have found an example at google books (unfortunately I cannot paste the link) which says:
One of the primary reasons why complementary and alternative approaches exist and persist is that we as veterinarians are not meeting, and often cannot meet, all of our client's needs. If clients ask us to reset...
The singular and the plural are used as if they were semantically equivalent (with "clients" meaning "a client, any client").