Is a person sinful if he does not prevent a wet dream while in a light sleep close to wakefulness, especially in sleep after Fajr, or should he surrender to that?

Wet dreams (احتلام) do not occur while awake, but rather during sleep. They are excused because the pen is lifted from the sleeper. If one wakes up and experiences a wet dream without any intervention on their part, there is no sin upon them. However, if one discharges semen through their own action, such as foreplay or similar acts, then it is considered masturbation and takes its ruling.