Will Allah accept the repentance of a person who committed a major sin, such as kissing a girl on her mouth and she kissed his penis? Has he, by doing so, committed a major sin? What are the supplications or actions through which Allah will accept his repentance?

Allah forgives every sin if the repentance is sincere. The conditions for repentance are three: abstaining from the sin, resolving not to return to it, and regretting having committed it. One who repents from sin is like one who has no sin. Allah forgives His servant and pardons him no matter how great his sin. The servant should frequently remember Allah and perform good deeds that efface sins, associate with the righteous, and acquire beneficial Islamic knowledge.