Is it permissible for this girl, who committed fornication and repented, to marry a young man who committed fornication with a foreign woman and had a child with her, a child he only now knows about and wants to reclaim to raise, knowing that she is willing to raise the child and he is certain of the child's lineage?

The child born out of wedlock is attributed to his mother and not to the fornicator, according to the majority of jurists. However, some jurists hold that if the fornicator acknowledges the child as his own, then the child's lineage is established with him. Based on this latter opinion, a man can attribute the child to his lineage, and he should strive to take the child from his mother to raise him properly. Your assistance in raising him after marriage will be greatly rewarded. Before marriage, consult trustworthy individuals about the young man, and then seek guidance from Allah the Almighty through Istikhara.