Not a matter of forgiveness
Lin Jing was not a cold-hearted mer who would ignore someone in trouble but the thing was that Gu Mu deserved it!
In fact, he would have helped Gu Mu if he was as foolish as Kong Tai and Gu Zedong who thought that Gu Mu was an obedient child and even though she was boisterous, she wouldn't hurt a fly.
Compared to them, it would be correct to say that Lin Jing was a very smart mer, even though Gu Zedong insisted on bringing Kong Tai home after she married him, he never refused her in fact he even allowed her to marry Kong Tai and start a family with him.
That was because he knew that a mer like Kong Tai who only knew how to rely on his wife or his daughter would never be able to raise a daughter as worthy as his Gu Ke Jin and sure enough his prediction came true.