In the absence of any evidence to the contrary, a civil court would rule that the recipient of a gift (including a wedding ring given by a spouse) held full title to it. i.e. that it's their property alone and that they're free to do what they want with it.
The same principle (that the giving of a gift transfers full title to it) would apply to the voluntary return of a ring by one partner to the other. i.e. once the woman had said "Here, take it" she lost all rights to the ring (and couldn't reclaim those rights through later changing her mind).
There's no chance that a charge of 'theft' could be brought anyway, since anyone who genuinely believes that they have the right to possess (or sell) property (even if that belief is ill-founded) can't be convicted of theft. [Section 2, Theft Act 1968]. So it's entirely a civil matter.