Be careful on this. If your relative isn't there all the time, the neighbor may have just been trying to help out (based on their perception of what needed to be done) and was not trying to steal the hay. Check with the neighbor (nicely) to see what was up.
I was gone for awhile and my neighbor took it upon himself to come over and mow (without letting me know). He ended up mowing down some highly prized flowers, but he was trying to look out for my best interests - we ended up being very good friends and I'd trade the flowers for the friendship.