Frank, as we all likely suspect, would know a lot! I'm loathe to criticise him too much though for not testifying/making a statement without "walking a mile in his shoes". I'd speculate that there was a huge power imbalance in the relationship he had with OP - who was not only a national hero but from an old and wealthy family. Is it unlikely/impossible that he felt intimidated? I'd say no.
I hate to say it but after following this case and especially hearing how people spoke of Mrs Stipp I would avoid testifying at a televised murder trial if there was any option to do so. I love my privacy. No bookface for me.