All perfect praise be to Allah, The Lord of The Worlds, and may His peace and blessings be upon our Prophet Mohammad and upon all his family and companions.
There are two cases regarding having intercourse with wife when she is menstruating:
First: The wife has mistakenly or forgetfully thought that her menstrual cycle ended (She has become clean), but it didn`t. In this situation, we pray that Allah, the Almighty, pardons them since the Prophet (PBUH) said: "Verily Allah has pardoned [or been lenient with] for me my ummah: their mistakes, their forgetfulness, and that which they have been forced to do under duress." {Related by Ibn Majah}. In addition, both are free from sin.
Second: Both have done so intentionally although they know that it is prohibited. In this case, both must make repentance and seek forgiveness. Al-Khateeb Ash-Shirbini {May Allah have mercy on him} said: "Deliberate intercourse with one`s wifein the natural place (vagina) during menses, although aware of the prohibition, is a major sin." {Moghni Al-Mohtajj, vol.1/pp.110}.
In the second case, it is recommended that the couple pay a golden Dinar (4,25 grams of 24 Carat pure gold) as expiation; if the intercourse took place at the beginning of the menstrual period, and half a golden Dinar if it happened at the end.
Ar-Ramli (May Allah have mercy on him) said: "It is recommended for the one who had intentionally had intercourse with his wife when she is menstruating; although both know that this is prohibited, to paya golden Dinar or its cash value, as an expiation,even to one poor Muslim person; if the intercourse took place at the beginning of the menstrual period, and half of that if it happened at the end." [Nihayat Al-Muhtaj, vol.1/pp.332]
In both cases, the wife doesn`t have to make Janabah Ghusl (Ghusl after discourse). Rather, it suffices that she makes menstruation Ghusl (Ghusl after menses).According to An-Nawawi: "If a woman was to make two Ghusls {Ghusl after menses and Ghusl after intercourse}, then one of them suffices." And Allah Knows Best.