The boyfriend will never be her true husband. Before God's eyes, the husband will always remain her true and only husband no matter what.
Catholics do not divorce nor they re-marry so they can have a new "chance" to be happy. That is worldly, secular thinking; not Catholic. As Catholics we must, as Christ commands, take up our crosses (in this case, an unfortunate marriage) and follow Him unconditionally.
She is allowed to physically separate from her husband if she is being physically or mentally abused and work towards a reconciliation, if that is possible. If not possible, then she must live a chaste and virtuous life apart from the husband, doing God's will. The purpose of marriage is the procreation and upbringing of children, not finding "happiness" on earth. That is a silly utopia.
Separation does not mean she is free to re-marry, unless she becomes a widow. Only then, she could re-marry. That is not cruel. Catholics are concerned with Heaven, where true life is found. This earthly exile is only temporary and we need to learn how to suffer meritoriously.
The war on marriage and the Traditional family really began the day the evil of Divorce was made legal. It should be reverted. Catholics are married for life, for better or worse.