In general I agree with the others that you have essentially been scammed by your ex and that it is for your to decide whether you want to take that to a legal level (which I would not recommend just because of the pain it would involve for a very little amount of money) or not.

That said, if making any change to the reservation, regardless of who paid for the ticket officially or not, your ex will have had to confirm (tick a box if online, confirm verbally if on the phone or in person) that he had your permission to make the changes. It is thus plain that he lied and claimed to have your authorisation while he did not. You could perhaps use that as leverage to ask him to refund the cost of your BA ticket.

Conversely, bear in mind that even with a non-refundable ticket, he may have reclaimed taxes and surcharges back on the unused ticket. Depending on how much that was, considering that there would be a processing fee, he may or may not have got much cash back.

So the short answer is that airlines do not have the responsibility to work as insurance policies against relationships getting sour and as such, VS seem to me to have gone well beyond their duty in reinstating your ticket. Your only possible case against BA would be to suggest that they did not take sufficient precautions to ensure that the passengers' authorisation was obtained by the people making the changes, but frankly, I think that it would be a very difficult case to make as beyond requiring passengers to know name and booking reference to make any change, they go beyond most airlines by asking for confirmation that the person making the changes is authorised to do so by all the passengers on the booking. In that sense, to me this is simply between you and your ex and becomes a question of how you want to take this further (or not) in the context. If it were me, I'd let it go and consider that my peace of mind comes from not having any further discussion with someone as bitter as you describe, but you are of course entitled to take a less lenient position in the sense that your ex unfairly cancelled a plane ticket that was wholly and completely yours.