I loved both of those series!
If you're looking at the OG Nancy Drew, I can't think of any that involve solving a murder, I didn't read as many Hardy Boys but I think they may have only had one or two if any.
Now there are a couple of new Nancy Drew (here) and Hardy Boys (here) series that are set in like 2004ish, they're really good books but very different from the OGs. The first few are individual, some of them are trilogies. There's murder in a few of the Nancy Drew and most of the Hardy Boys. I'd wager that the newer Hardy Boys would be the best way to get him started.