Well I would be royally ticked if as the stallion owner if this was done on purpose, not sure how I'd handle this situation, probably just suck it up and move on and find ways to prevent this type of thing in the future. Mostly because, like was said, it's all hearsay.
Technically it would be the mare owner's fault if the mare got out and got bred, BUT the mare owner is the one that suffers when they end up w/ a potentially unwanted foal. The stallion owner may see themselves out the stud fee, but if it was an accident, that's probably not a stud fee they were going to get anyway and no one is happy. I'd be very unhappy and ticked if my mare ended up bred, I don't care who's to blame (but blame will go where it should, if it's my fault I'll own up to it). Now when registration is involved, if it's the stallion owner's fault they should sign the papers, if it's the mare owners fault they should have to pay the stud fee to get papers.
Now there is a difference between an accident and an owner slipping their mare in w/ the stallion when no one is around (I wasn't there, but it sounds like that may be the case here).