What is the ruling on shunning a Muslim friend for five months due to jealousy over his engagement to the same girl his friend desired, and what should he do now with his question about whether Allah will forgive him for that?

The basic rule is that it is forbidden to abandon a Muslim for more than three days, unless there is a legitimate reason, such as fearing harm from their company. However, abandoning a Muslim simply because of love may not have a legitimate religious justification.