About Divorce: Please look at the bottom of this page.
Issue 2548: If a woman lays a condition at the time of Nikah that if her husband goes on a journey or, for example, does not give her maintenance for six months, she will have the right of divorce, the condition is void. However, if she lays a condition that if her husband goes on a journey or, for example, does not give her maintenance for six months, she will be his Wakil for her own divorce, the condition is in order.
It says woman would have the right to divorce if her husband won't support her financially for 6 months. I think 8 years is a pretty long time to have this right. Doesn't matter if nikah is makruh or not, I suggest her to divorce from that unresponsible man.
I searched for the women whose marriage is haram with (from male's perspective) and found these verses (Surat An-Nisa' 4/ 22-23-24-25):
And do not marry those [women] whom your fathers married, except what has already occurred. Indeed, it was an immorality and hateful [to Allah ] and was evil as a way.
Prohibited to you [for marriage] are your mothers, your daughters, your sisters, your father's sisters, your mother's sisters, your brother's daughters, your sister's daughters, your [milk] mothers who nursed you, your sisters through nursing, your wives' mothers, and your step-daughters under your guardianship [born] of your wives unto whom you have gone in. But if you have not gone in unto them, there is no sin upon you. And [also prohibited are] the wives of your sons who are from your [own] loins, and that you take [in marriage] two sisters simultaneously, except for what has already occurred. Indeed, Allah is ever Forgiving and Merciful.
And [also prohibited to you are all] married women except those your right hands possess. [This is] the decree of Allah upon you. And lawful to you are [all others] beyond these, [provided] that you seek them [in marriage] with [gifts from] your property, desiring chastity, not unlawful sexual intercourse. So for whatever you enjoy [of marriage] from them, give them their due compensation as an obligation. And there is no blame upon you for what you mutually agree to beyond the obligation. Indeed, Allah is ever Knowing and Wise.
And whoever among you cannot [find] the means to marry free, believing women, then [he may marry] from those whom your right hands possess of believing slave girls. And Allah is most knowing about your faith. You [believers] are of one another. So marry them with the permission of their people and give them their due compensation according to what is acceptable. [They should be] chaste, neither [of] those who commit unlawful intercourse randomly nor those who take [secret] lovers. But once they are sheltered in marriage, if they should commit adultery, then for them is half the punishment for free [unmarried] women. This [allowance] is for him among you who fears sin, but to be patient is better for you. And Allah is Forgiving and Merciful.
And also you can look into these verses for prohibition of marriage with a non-Muslim:
And do not marry polytheistic women until they believe. And a believing slave woman is better than a polytheist, even though she might please you. And do not marry polytheistic men [to your women] until they believe. And a believing slave is better than a polytheist, even though he might please you. Those invite [you] to the Fire, but Allah invites to Paradise and to forgiveness, by His permission. And He makes clear His verses to the people that perhaps they may remember. (Surat Al-Baqarah 2/221)
O you who have believed, when the believing women come to you as emigrants, examine them. Allah is most knowing as to their faith. And if you know them to be believers, then do not return them to the disbelievers; they are not lawful [wives] for them, nor are they lawful [husbands] for them. But give the disbelievers what they have spent. And there is no blame upon you if you marry them when you have given them their due compensation. And hold not to marriage bonds with disbelieving women, but ask for what you have spent and let them ask for what they have spent. That is the judgement of Allah ; He judges between you. And Allah is Knowing and Wise. (Surat Al-Mumtahanah 60/10)
Conclusion: Her situation (brother's wife) doesn't fit any of these above. So, according to Qur'an, she seems to be allowed to marry with her husband's brother, after divorce, legally etc of course.
I don't know which country she is from, thus traditions and thoughts of her society. But, I think she should be careful about them, not for religion, but for social consequences. In some parts of the world, marrying with ex-husband's brother won't be approved, while in some parts of the world it would be supported. But it seems to be no obstacles in a religious way.