Nigel,
There is nothing particularly unsafe about what the crew did, I think. The thermal runaway for a NiCad is not triggered by charging a flat battery, it is usually caused by massive discharge (PPRuners, please jump in and correct me if appropriate. Even if not appropriate!).
It would be good practice to charge the battery with a careful trickle charge, at constant amperage like a battery shop does it, rather than at constant voltage as in most aircraft. There is nothing special about the S-76 this way, it is similar to most helos in this aspect. Black Hawk has a conditioner analyzer circuit for the battery which does a trickle charge technique.
This is something where we can learn from others, too. Ppruners, please pipe up!
Nick