Assalamu alaikum wa rahmatulahi wa barakatuh,
Alhamdulilah that your intentions for her are good in that you want to encourage her to do things pleasing to Allah, but having a relationship outside of marriage, even as friends like this is not something that will be pleasing to Allah, hence why He has made the results of such haram and prescribed marriage to meet the needs of such relationships and this is what pleases him. Therefore, the first step in encouraging her to do good is to remove the temptations of this relationship from her until you are able to seek marriage.
The first thing I would advise is to refrain from direct contact with her, even via non face to face means such as WhatsApp and Snapchat. This might seem quite harsh since it seems that you have already developed feelings enough for her that you would like to marry her at some point in the future, but there are many good reasons for this.
Keep in mind that the hijab that you would like to see her observe, along with modesty, is not being encouraged by having this kind of relationship with a member of the opposite sex such as you, so by having such contact you are automatically contributing to her not observing modesty and the general sense of the word hijab beyond it being simply a covering of the hair, but a covering beyond this to both physical and social modesty.
As you have observed first hand, talking to her and getting to know her has lead to the development of romantic feelings. These feelings can easily lead to haram via fornication, zina and even adultery. As humans we have desires, but we are also not so strong at controlling our desires. When it comes to romance and intimacy, Allah has encouraged us to get married to fulfil many of these desires. However, fulfillment of these desires outside of marriage is a sin and this is why marriage is encouraged as a protection for society to prevent things such as children being born outside of marriage and rights not being fulfilled. There is much wisdom and benefit in marriage that can cannot be attained through non marital relationships.
It is important to understand this wisdom to understand why having relationships with the opposite sex come with many risks and is why contact with the opposite sex who you are bit a mahram for should also be restricted to avoid the development of feelings that can easily lead to sin. You may feel you have control over these things and that you won’t commit sin, but controlling the nafs is never an easy task and jihad of the self like this can be very challenging. Allah knows us better than we know ourselves and this is why he prescribed marriage in order that we don’t have to face what we are not strong enough to.
Therefore, I would suggest that you either move forward in getting married soon so that you can resume such contact, or, if you don’t plan to get married anytime soon then either cut contact completely until then, or if you want to be in touch make sure there is a third party present to prevent you both falling in to sin, even via online contact. This can be done by adding her mahram to the WhatsApp conversation for example so they can monitor your conversation. It may seem like a strange thing to do, but even one to one contact online can lead to inappropriate interactions, in fact, even more so than face to face as when people talk online, they are behind a veil which leads to losing one’s inhibitions to some extent which can lead to sin even more easily than face to face contact.
Beyond your current situation, once married you would be more easily able to directly influence her behaviours and encourage her to do things that please Allah, such as observing physical modesty and wearing hijab. She would also be less inclined to engage with other men as she has a spouse and this would protect her modesty more generally. For now, you could instead take indirect steps by asking those that you are permitted to talk to to talk to her and encourage good in her. This maybe less direct by asking them to invite her along to local religious events and classes in the community that will encourage her to keep good company with those who do good deeds and conduct themselves well and therefore will nurture her to do the same. This way they have to necessarily make direct comments about the way she dresses for example and will encourage her to make these changes herself of her own volition. In sha Allah, by the time you are ready to wed her she will have already established these things.
May Allah guide you and this girl to the things that He is pleased with and grant you happiness and success in both this life and the next.
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