Buddhism does not usually speak of an eternal divine refusal to forgive a “sin.” There is no creator God who permanently withholds pardon. Instead, Buddhist ethics focuses on karma: intentional actions have consequences, and especially grave actions are described as producing severe karmic results.

Grave actions are not trivialized
Some Buddhist traditions identify five extremely serious acts, including killing one’s mother, killing one’s father, killing an arahant, maliciously injuring a Buddha, and creating a schism in the Saṅgha. These acts are treated as exceptionally weighty because of their intention, object, and consequences.
Saying Buddhism has “no unforgivable sin” can therefore be misleading if it suggests that harmful actions do not matter.
Forgiveness and karma are different ideas
A person may be forgiven by someone they harmed, confess wrongdoing before a community, or cultivate remorse. None of those acts simply erases all karmic consequences in Buddhist thought.
Karma is not a punishment administered by a divine judge. It is a moral pattern of intentional action and consequence, interpreted in somewhat different ways across Buddhist schools.
Remorse without despair
Healthy remorse recognizes harm and motivates change. Buddhist practice generally distinguishes this from paralyzing guilt or the belief that a person is permanently evil.
Practices can include confession, making amends when possible, taking precepts again, generosity, meditation, and deliberately cultivating wholesome mental states.
Transformation remains meaningful
Buddhist stories often emphasize that people with harmful pasts can change direction. The importance of present intention means ethical practice remains meaningful even when past actions cannot be undone.
That does not guarantee escape from every consequence; it means moral development and restraint are still possible.
The Buddhist framing
A better question than “Can God forgive this?” is “What consequences follow from this action, and how can the person stop harm and cultivate a different path?”