Is it obligatory to inform a suitor about the past of a girl I know who had a previous relationship, or is that considered exposing her?

If the girl repents and becomes righteous, no one should be informed of what she used to do. However, if she persists in evil and does not repent, the suitor must be informed of her condition.

It is permissible to generally advise against inquiring about her condition without going into detail. To understand the regulations regarding informing a suitor about a prospective bride's faults, one can refer to the mentioned fatwas.