Bass' conversation with Solomon is shortly before his release. Considering he was captured sometime in 1841, that would put their conversation sometime around 1852-3...
In 1850, revisions were made to the Fugitive Slave Act of 1793, basically sanctioning harsher punishments for escaping slaves but also denigrating the character of those who would seek to help them, in response to the underground railroad which reached its height at this period.
This alone is nothing to worrisome for a white man, but it reflects the present mood and anxieties of the time. The Law was intended to threaten and intimidate those who would help slaves, and turn public opinion entirely against them..
New Orleans was a Slave-State, and it would have been difficult for Bass to survive in a slave state if he was outed fully as a sympathizer: not just in spirit, as his conversation with Epps, but in action by facilitating Epps' loss of property. He would be ostracized at best (and thus denied his livelihood),but more likely harangued and possibly subjected to violence...
It was a real risk for Bass to help Northup. Even though all he was required to do in actuality was sent a letter to Saratoga Springs, if he was caught or it was found to be him sending it, the ramifications would be serious...