I note that you posted only snippets of the CCC.
Well, here are some full statements posted in their entirety and it shows that you again are wrong.
2384 Divorce is a grave offense against the natural law. It claims to break the contract, to which the spouses freely consented, to live with each other till death. Divorce does injury to the covenant of salvation, of which sacramental marriage is the sign. Contracting a new union, even if it is recognized by civil law, adds to the gravity of the rupture: the remarried spouse is then in a situation of public and permanent adultery:
If a husband, separated from his wife, approaches another woman, he is an adulterer because he makes that woman commit adultery, and the woman who lives with him is an adulteress, because she has drawn another's husband to herself.177
2385 Divorce is immoral also because it introduces disorder into the family and into society. This disorder brings grave harm to the deserted spouse, to children traumatized by the separation of their parents and often torn between them, and because of its contagious effect which makes it truly a plague on society.
The CCC states that it IS a grave offense, contrary to your claim that it's not.
It also states that it is immoral, which makes it a sin.
Generally, divorce is immoral, but not always. This is why the Catechism gives example when it is “tolerated” in 2383, already cited. Like most things.
This woman doesn’t have to divorce her husband. What she needs to do is separate from him till such time their marriage can be consecrated in the Church. Had she done that, her sinful condition would have stopped and she would have received her absolution.