The best option in my opinion is to interrupt the process any time you wish. You are correct that only securely deleting the user account is faster than securely deleting all unused space (except for the edge case where the drive it 100% totally full except for this user's data - unlikely in practice and at risk of failing due to no swap space being available). You will have added time to re-do the erase, so unless you are certain it's frozen, letting it run might be the best course until you need to interrupt the process.
Any time you have to halt the mac, there could be some damage to the files being written, but at this point, the mac is simply wiping the user files that are about to be deleted.
If you have a recent backup or bootable restore media, you are really in great shape. Once the mac reboots, you will of course have to deal with the complete deletion of the /Users/whatever folder and potentially need to re-do the user deletion if it's still partially left around. You can still choose the secure delete command but this time have a user logged in to inspect the progress. You can also insecurely delete and either skip that security or have the mac run a secure overwrite of all free space when you can afford the time to write over all the free space to ensure security if someone were to try to search for files among the unallocated space.
I would let the mac delete as long as you can, and then ask back a new question linking to this if you should run into any questions or errors on cleanup.