A flag displayed upside down is a internationally recognized sign of "dire distress" and "extreme danger to life or property".
People who have used the upside down flag as a symbol of protest have muddied the true meaning.
A sideways cross can be an artistic interpretation or religious significance.
The sideways cross is used on graves is the hope of the resurrection.
The problem is that people who see her wearing it may not be aware, and they may find it offensive as well. I don't think it is internationally known and accepted. I would not be comfortable wearing one, nor would I find it really pretty on someone else to be honest. (I work part-time as an organist along with my main job as a paralegal).
If you question whether or not it would be offensive and are not sure about how she feels about the sideways cross, then I wouldn't buy her one. Unless you know that she either wants one or would value the significance of it, I'd stick with a tradtional cross.