Crypto currency capital gain?

You could input a summary of the 8949 and attach a copy to the return. https://proconnect.intuit.com/community/form-8949/help/attach-a-summary-statement-to-schedule-d-form-8949/00/5333

I would enter the taxpayer name and SSN on the 8949 provided. The result is just like any other "stock" transaction. Verify that every page of the 8949 you received has the same A, B, C, D, E or F box and enter the summary to match the letter indicated. 

Be sure the box on page 1 of the 1040 is marked YES. You can find the entry point by looking at the Check Return and finding the box on the 1040. Click that box and enter the appropriate code. 

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.