It takes extremely bad mental health to even consider suicide and that is very complex and not just simply caused by a cheating partner.
Someone convincing you you've gone mad and are being paranoid, anxiety about being pushed away by a partner etc etc - there are so many ways this kind of thing can affect the unknowing partner once they know, unfaithfulness in itself it's a complex issue.
People can absolutely become suicidal due to the complex and multifaceted ways an affair can affect them. It grinds them down, they lose their confidence, self worth, intimacy and often their confidant.
People fall out of love all the time, that's just real life. If someone meets someone and they never knew what they were missing but side you realise they were missing it then that's life. But you leave. You don't skulk around behind your partners back and make them feel shit. You don't lie to their face when questioned about it. You don't do all that and then say well they obviously weren't stable anyway if they feel suicidal. No, perhaps they didn't feel suicidal until the complex and devastating consequences of the behaviours during an affair. It's not the leaving that fucks up the person cheated on, it's the lies and betrayal.
If people want to be with someone else then it's so unbelievably selfish to have an affair rather than being brave and leaving their partner to become healthy and successful coparents instead of being deceitful.