No, if a woman reports the advances of a non-mahram (or a mahram for that matter) to her husband, this is not considered backbiting (whether doing so is advisable is a different matter). Allah has defined the exception:
لَّا يُحِبُّ اللَّهُ الْجَهْرَ بِالسُّوءِ مِنَ الْقَوْلِ إِلَّا مَن ظُلِمَ وَكَانَ اللَّهُ سَمِيعًا عَلِيمًا
Allah does not like that the evil should be uttered in public except by him who has been wronged. And Allah is Ever All-Hearer, All-Knower.
— Surat An-Nisa' 4:148
At-Tabari in his tafsir of this verse said Allah does not like an evil being publicly uttered except for those who have been through injustice, in which case there is no harm in mentioning what took place, and the person receiving the injustice is permitted to retaliate fairly. The same view was shared by Mujāhid, As-Suddi, and others.
This is why jurists generally tend to exempt injustice from being considered backbiting: recounting the incident of injustice, supplicating against the transgressor, warning people about the transgressor's attitude, retaliating in kind, etc. (refer to backbiting about a non-muslim and complaining to seek advice for more information).
More information is below for reading at your own leisure.
Ghībah (Arabic: غيبة) is commonly referred to as backbiting, which is mentioning about a Muslim truth(s), but doing so behind their back while they would hate to have heard so in person or would prefer to keep such matters confidential.
Namīmah (Arabic: نميمة) is commonly referred to as gossip, which is telling one what others have said about one in order to damage the relationship between them.
Buhtān (Arabic: بهتان) is commonly referred to as slander, which is mentioning about a Muslim untruth(s) in order to damage their reputation.
What constitutes ghībah
The Prophet ﷺ laid out the basic definitions of ghībah in this hadith:
عَنْ أَبِي هُرَيْرَةَ، أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم قَالَ: أَتَدْرُونَ مَا الْغِيبَةُ. قَالُوا: اللَّهُ وَرَسُولُهُ أَعْلَمُ. قَالَ: ذِكْرُكَ أَخَاكَ بِمَا يَكْرَهُ. قِيلَ: أَفَرَأَيْتَ إِنْ كَانَ فِي أَخِي مَا أَقُولُ. قَالَ: إِنْ كَانَ فِيهِ مَا تَقُولُ فَقَدِ اغْتَبْتَهُ وَإِنْ لَمْ يَكُنْ فِيهِ فَقَدْ بَهَتَّهُ
Abu Huraira reported Allah's Messenger (ﷺ) as saying: "Do you know what is backbiting?" They (the Companions) said: "Allah and His Messenger know best." Thereupon he (the Holy Prophet) said: "Backbiting implies your talking about your brother in a manner which he does not like." It was said to him: "What is your opinion about this that if I actually find (that failing) in my brother which I made a mention of?" He said: "If (that failing) is actually found (in him) what you assert, you, in fact, backbit him, and if that is not in him, it is a slander."
— Sahih Muslim, Book 45, Hadith 91
As per Al-Athkar by Yahya ibn Sharaf An-Nawawi (Beirut, Lebanon: Dar Al-Fikr, 1994, pp. 336-337):
اعلم أن هاتين الخصلتين من أقبح القبائح وأكثرها انتشاراً في الناس حتى ما يسلمُ منهما إلا القليل من الناس فلعموم الحاجة إلى التحذير منهما بدأتُ بهما.
فأما الغيبة: فهي ذكرُك الإِنسانَ بما فيه مما يكره سواء كان في بدنه أو دينه أو دنياه أو نفسه أو خَلقه أو خُلقه أو ماله أو ولده أو والده أو زوجه أو خادمه أو مملوكه أو عمامته أو ثوبه أو مشيته وحركته وبشاشته وخلاعته وعبوسه وطلاقته أو غير ذلك مما يتعلق به سواء ذكرته بلفظك أو كتابك أو رمزتَ أو أشرتَ إليه بعينك أو يدك أو رأسك أو نحو ذلك.
— NOTE: My own translation, so treat with care:
Be informed that these two habits [ghībah and namīmah] are one of the ugliest and most widespread sins among people; only a few people are saved from them. Accordingly, due to the general need to warn against them, I started with them.
As for backbiting (ghībah, Arabic: غيبة), it is mentioning a person with what they hate, let that be about one's body, religion, life, oneself, creation, manners, wealth, children, parents, wife, servant, slave, turban, attire, way of walking, way of moving, happy countenances, sad countenances, frowning, fluency, or any other thing related to one. This applies whether [ghībah] is mentioned using direct words, in writing, figures of speech, or other forms of insinuation through the eyes, hands, head, etc.
He elaborated with examples, some of which are below for your reference:
- Body. Calling someone blind, short, fat, black, yellow, leper, etc., that is said in a derogatory or belittling manner.
- Religion. Calling someone immoral, defiant, careless about prayers, disobedient, hypocrite, disbeliever, careless about impurities, etc.
- Life. Calling someone talkative, ignorant, sleeps a lot, eats a lot, etc.
- Parents. Calling someone's parents immoral, belittling them based on race or color or profession, etc.
- Manners. Calling some bad-mannered, arrogant, rushed, brown-noser, ill-tempered, etc.
- Attire. Calling someone's attire as too short, too loose, dirty, etc.
What is not considered ghībah
Scholars have mentioned several situations where talking about one behind one's back is not considered ghība that can be classified into four categories:
Identification. This is when mentioning something by which a person is commonly identified (e.g., Surat 'Abas 80:2 refers to Ibn Um Maktūm as the blind man), long as the intention is not to belittle one.
When wronged. When wronged, one is allowed to complain, recount, supplicate, seek advice, seek fatwa, seek help to stop the injustice, etc.
To warn others. Warning other Muslims about a person's behavior, attitude, actions, deviations or innovations in religion, wrong knowledge or information or data, etc. (e.g., studying people on narration chains of hadiths and deciding on the grade of the hadiths based accordingly is not considered ghībah).
To give advice or criticize. Mentioning what a person hates when giving advice or criticism is not considered ghībah.