Googling someone isn’t a bad idea, if that is something that enables you to feel safer about the person you just met on a dating site.
However, discovering that the person is married and messaging the person’s spouse, is not the best course of action.
Report the person to the dating site. Let them know that you discovered that the person is married.
The dating site will do its due diligence from that point on and hopefully remove the person’s dating profile from the site. That should be the extent of your involvement in this.
You had a difficult past experience in having to deal with a cheating spouse that you didn’t know was cheating on you until it was too late, but don’t let that emotion cloud your judgment.
Your objective is to find a partner on a dating site, not to become the cheating spouse police. Report the person to the dating site and let them handle it from there.
Stay focused on your purpose.
If you start trying to send messages to spouses who have cheating spouses that are using dating sites, you could enter a rabbit hole that is hard to extricate yourself from.
Their marriage is none of your business.
Again, report the dating profile to the dating site and let them handle it. Keep your purpose on finding the right partner for you.