Be very careful how you report these transactions - conceptually, it's the same as securities trading but they could be much more complex in practice due to the lack of centralized records and the ability to trade in fractional units promoting more frequent trading.

These could get dicey, especially if your client trades on different exchanges, have crypto-to-crypto trades, and doesn't use any platform to track the bases.

In case your client has voluminous transactions but can't provide you with clean records you could reasonable rely on, you may consider asking him/her to engage one of those tax service providers who specialize in crypto-traders instead as they would be better equipped to deal with this type of returns.