Titanium and Niobium are just fine for rings. (Structurally, anyway.
They’re so soft they’ll scratch to pieces in no time, but that’s not
a structural consideration.)
What caused the problem in your husband’s case was panic and a poorly
equipped hospital, not the metals involved. They would have had just
as much trouble with that 18K ring with or without the niobium
Neither titanium nor niobium are particularly hard, or difficult to
cut with hand tools. Titanium, at least, has a nasty rep because it
has been marketed as some sort of aerospace “super metal”. It’s not,
but it sure makes the marketing easier if people think it is.
Niobium cuts about like (gummy) brass, and most titanium alloys are
on par with stainless steel. Both of them are much softer than 14K
nickel white gold.
As far as fixing your husband’s ring, there are a couple of people
here on orchid who could probably remake it for you.