On Sun, Feb 7, 2016 at 7:03 PM, Patrick Strateman via bitcoin-dev <bitcoin-dev@lists.linuxfoundation.org> wrote:

I would expect that custodians who fail to produce coins on both sides
of a fork in response to depositor requests will find themselves in
serious legal trouble.

If the exchange uses an UTXO from before the fork to pay their clients, then they are guaranteed to count as paying on all forks. The exchange doesn't need to specifically pay out for each fork.

As long as the exchange doesn't accidently double spend an output, even change addresses are valid.

It is handling post-fork deposits where the problem can occur. If they only receive coins on one fork, then that should cause the client to be credited with funds on both forks.

The easiest thing would be to refuse to accept deposits for a while before/after the fork happens.