All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.
If the marriage of that woman was conducted fulfilling the conditions of the valid Islamic marriage, i.e. having the Wali (the guardian of the woman) and two witnesses, then the marriage is valid, even if she discovered that her husband is unchaste. For more benefit, kindly refer to Fatwa 83176.
This is the ruling if he is proved to be unchaste, but how can she be sure of that? Photos and messages alone cannot in anyway mean that the man is an adulterer and it is not permissible for a wife to think evil of her husband as long as there is no proof for that. Allaah The Almighty says (what means): {O you who have believed, avoid much [negative] assumption. Indeed, some assumption is sin. And do not spy or backbite each other. Would one of you like to eat the flesh of his brother when dead? You would detest it. And fear Allaah; indeed, Allaah is Accepting of repentance and Merciful.}[Quran 49:12]
In most cases, bad assumption leads to spying which is prohibited as well. This may be the reason for mentioning it after the bad assumption (in the above-mentioned verse). For more benefit, please refer to Fatwa 89109.
However, if that husband is proved to be unchaste and he did not repent from that, then his wife has the right to ask for divorce from him; rather asking for divorce is most appropriate in that case. If he divorces her and her waiting period comes to an end, then it is permissible for her to marry another man. For more benefit, please refer to Fataawa 85390 and 90481 about the valid reasons for asking for divorce.
As for the debt he owes her, then she can ask him to pay it off to her whenever she wills whether he divorces her or not; however, if there is a fixed term for that debt that has not yet passed, then the term should be observed.
Allaah Knows best.