1 This is tricky, both for HF (his involvement with drug dealing leaves him vulnerable to all kinds of ramifications of testifying, and one question on this can cause another on that) and the court (it's hard to properly limit scope of questions, in a way that can protect his rights and get him to testify)
2 "Refusal to testify" cannot legally be taken as an indication of self-incrimination, or inferred as such, and the judge will likely inform the jury of that
3 Without his testimony to refute it, state will have a hard time reining in the wild speculative questions for others, that infer his connection in some way with no real evidentiary basis.
4 IMO this will make the trial run longer, because easier for defense to create rabbit trails that the state will have to erase, and more need to keep refocusing jury on the evidence (she was put IN HIS TRUNK) itself
5 Even if the state knew a month ago, they had to bring him to court anyhow and get the refusal and see where it goes. No real alternative. Fortunately, not a surprise, so must have been prepared.