While this is a very sad story, it doesn't change the fact that prostitution is illegal. Even if it helped save her life, it's still illegal.

Until it is made legal (not happening in the US), there is no argument that should work in favor of making it easier.

There are reasons prostitution is illegal. There are reasons to make it as difficult as possible for someone to find a prostitute. There are enough stories of people who tear apart their lives and families because it was too easy to find a prostitute (implication is that they wouldn't have used a prostitute if they couldn't find one easily and quickly).

It's sad that she suffers from bi-polar, but this is not a utilitarian society. Backpage helping her doesn't justify the fact that it directly led to a proliferation of illegal activity.