Ser Jorah is not the only person in Daenerys' life who is involved in the slave trade. Just about all authority figures she knows are, such as Magister Illyrio, Khal Drogo (or any Dothraki), or Xaro Xhoan Daxos. Slavery is a common occurrence in Essos, though forbidden in many cities, such as Pentos and Braavos.
As I recall, Daenerys says something like that it was only in Astapor that her eyes were opened to the cruelty of slavery, and I think that she is not a fanatic about slavery to the point that she hates anyone ever involved in it. Also, Ser Jorah did what he did because of love (for his wife Lynesse), not because he was a slaver.
The question is valid, though, as one could easily make the argument that Daenerys had no problem with slavery until she had to pay for it. If she had, had the money, she might not have started the war or even spoken out against slavery. Where she is today is greatly due to necessity, rather than conviction.
Slavery is also a custom of Old Valyria, but not, it seems, a tradition the royal Targaryen family (who came from Valyria) upheld when they took Westeros. In that sense, I guess Daenerys is a true dragon, upholding her family legacy. But in a sense, Daenerys just jumped on a cause to get what she wanted, and now she finds that it is also a way to gain allies.