Is her marriage valid after she had an illicit relationship and became pregnant by someone other than her husband, then aborted the child? And how does this affect her child from an Islamic perspective?

First: If the adultery occurred after the marriage contract, then the marriage is valid, and you must repent. Second: If the adultery occurred before the marriage contract, the contract is not valid until the womb is cleared by one menstrual period, according to the preponderant opinion, or by three menstrual periods. Third: Given the considerable difference of opinion on the matter and the fear of fitna (strife/temptation), you are not obligated to inform your husband of what happened. Conceal your deed, for the Prophet, peace and blessings be upon him, said: "All of my Ummah are pardoned except those who make their sins public." Fourth: Your current son is legitimate to your husband. If the fetus you aborted was before the ensoulment, there is no expiation for it except repentance. If it was after ensoulment, then you owe the blood money (diyah) and expiation.