On this page :

http://docs.mongodb.org/manual/reference/limits/#limits-sharding

We quote the following limitation:

Sharding Existing Collection Data Size
For existing collections that hold documents, MongoDB supports enabling sharding on any collections that contains less than 256 gigabytes of data. MongoDB may be able to shard collections with as many as 400 gigabytes depending on the distribution of document sizes. The precise size of the limitation is a function of the chunk size and the data size.

IMPORTANT
Sharded collections may have any size, after successfully enabling sharding.

We don't really explain the rational for this limitation nor do we give any guidance as to what a customer should do if they find themselves in this situation.

Might be worth suggesting they contact MongoDB if they find themselves in this situation.