Addiction is a brain disorder characterized by compulsive engagement in rewarding stimuli despite adverse consequences. Despite the involvement of a number of psychosocial factors, a biological process – one which is induced by repeated exposure to an addictive stimulus – is the core pathology that drives the development and maintenance of an addiction. The two properties that characterize all addictive stimuli are that they are reinforcing (i.e., they increase the likelihood that a person will seek repeated exposure to them) and intrinsically rewarding (i.e., they are perceived as being inherently positive, desirable, and pleasurable). Addiction is a disorder of the brain's reward system which arises through transcriptional and epigenetic mechanisms and develops over time from chronically high levels of exposure to an addictive stimulus (e.g., eating food, the use of cocaine, engagement in sexual activity, participation in high-thrill cultural activities such as gambling, etc.).
Sorry to hear you are going through that challenge.
My suggestion is to look online for some support groups for other people who are in a similar situation and see if you find any benefit from the advice and experiences they share.
Addiction is caused by weak will power in front of bad desire and can be tackled by strengthening the will power. Fasting is one of the good ways to strengthen the will power as well as practicing useful acts which are against the desire. It is helpful to repeat Estighfaar (ASTAGHFIRULLAH) and SALWAAT (ALLAHUMMA SALLI 'ALA MUHAMMAD WA AALI MUHAMMAD) and (LAA HAWLA WALA QOWWATA ILLA BILLAH).
The Ulama did not find in consuming betel nut any damaging harm to the health, that is why we don't see a verdict against it. Some materials which are mixed with betel nut can be very harmful like tobacco. Consuming any item which damagingly harmful must be avoided.
Spying is not allowed in Islam, but if you have come to know that your husband is having a bad habit, you need to do your best to help him to get out this bad habit in every possible way. You should explain to him the damages of such sinful acts and harms on him in this life and hereafter.
The root of these sinful acts is weakness in front of the sinful desires. This weakness can be tackled by:
1. Remembering the fact the Allah is seeing us always. Will we ignore His presence and commit the sinful act in front of Allah?
2. Remembering the harm and damage caused to your heart, mind and body by these sinful acts. How can a sensible person harm himself deliberately for the sake of few moments of sinful lust?
3. Remembering the fate of those who committed such sinful acts and how they gained nothing, but the loss and damage to themselves.
4. Remembering the great reward of pious people who keep themselves away from the sinful desires for the sake of Allah. The reward in Paradise is much greater than any lust.
5. Remembering the painful results and sever misery of those who committed sinful acts. Can you tolerate such dangerous results? Is this sinful desire worth putting you in such fatal results?
6. Repeating Estighfaar and Salawaat is very useful in cleaning the inner and enlightening the mind and heart.
7. Seeking the help from Allah with the Waseela of His most beloved servants, Muhammad and his Holy Progeny Ahlul Bayt (AS).