If one parent has beta thalassemia, what are the chances that their baby could inherit the condition?

Depends. If one parent has beta thalassemia major, and the other parent has none, then their children should all be carriers. If the other parent is a carrier then about half their children will be carriers and half have disease. If one parent has beta thal minor (carrier), and the other parent has none, then about half their children will be carriers and half will be normal. A geneticist can give details.
Thalassemia major & Intermedia are autosomal recessive (both copies of hbb gene in each cell have mutations). Each parent carries 1 copy of the mutated gene but typically doesn't show signs/symptoms. Sometimes people w/ only 1 hbb gene mutation develop mild anemia (thalassemia minor). In a small % of families, the hbb gene mutation is inherited as autosomal dominant meaning 1 gene copy causes condition.