How should one act concerning the desire to marry a girl who has repented and changed, but a friend of the fiancé had previously seen her body, knowing that the fiancé prayed Istikhara and felt at ease?

Whoever falls into sin must repent and conceal their own faults. The responsible person should not disclose the sins that Allah has concealed for them. If a girl repents with sincere repentance, there is no harm in marrying her, and she should not be abandoned because of her past sins, for repentance erases what came before it. The preponderant opinion is that the one who performs Istikhara should proceed with the matter after Istikhara, unless Allah diverts them from it.