A "working girl" I am friendly with occasionally takes a drug available from local pharmacy's that delays the onset of her period by a month. She says she was told it was safe as long as she didn't use it two months in a row.

I would imagine it is a pretty powerful hormone. Just how safe is it and how frequently can it be used?

Is it legal in the USA or EU?

It is imposisble to answer this without knowing what, actually, she is taking.

This effect can be achieved simply by takming ordinary birth control pills cotinuously (i.e. with no week off as is usual), either indefinitely or for whatever time period one wants to be menstration-free for. As far as is known (and there ahve been several studies on it) this is pefectly safe. The bleeding that occurs in users of the "Pill" is not real menstruation anyhow, it is just "withdrawal bleeding" due to a drop in hormone levels.

But whether this is what she is using, who knows.