From TFA "The FBI has not, however, been able to identify the hitman as yet. The reason is almost certainly because the hitman did not use a commercial service like Coinbase, which must comply with know-your-customer laws, for the Bitcoin transactions but instead relied on his own personal wallet—one that did not reveal any information about his identity."
If they try to convert to fiat via an exchange, they can be identified the same way the husband was. If they try to buy products directly without using fiat then it is harder for law enforcement to get the records they need, but still quite possible.
They will probably be safe if they never touch the bitcoin, but that entirely defeats the purpose of using it
An asumption you're making and likely untrue. There are many ways to convert to fiat or skip that step and luander through the dark web
Usually, one'd use a coin like Monero instead, the project's wretched history notwithstanding.
I can see a lot of value in it though, just like how I see value in Freenet, I2P, Tor and other such overlay networks.
(not implying that the bbc gave him any money)