Is the young man who promised a girl marriage online then left her, leading to her suicide, sinful, or does the sin of suicide rest solely with the girl?

This young man is not responsible for the girl's suicide, because his refusal to marry her does not usually lead to her killing herself.

However, he is unjust to her in two ways:

First: He lured her into a forbidden relationship and helped her commit a sin.

Second: He promised to marry her then broke his promise, and lying and breaking promises are characteristics of a hypocrite.

The young man must repent to Allah, abandon forbidden relationships, and start a new chapter of righteousness.