The root cause of this incident wasn't some flaw in Google's security. The root cause was that the victim's computer was breached, and that is the only way a threat actor can steal your session cookie. So besides all the things you already mentioned as security precautions, there are some quite effective approaches to combat these types of attacks:
1.) Use a proper, modern endpoint protection solution. I am not talking about your Symantec or Norton antivirus here, but something way more sophisticated: EDR. Endpoint detection and response. Think of this as next-gen antivirus that works entirely differently than traditional AV systems and is way more capable of detecting and stopping breaches happening on your computer. Some notable vendors in this area are SentinelOne, CrowdStrike or Cynet (amongst others).
2.) The number one attack surface for these attacks to start is email. 99% of all attacks start with a malicious email, typically phishing. So, deploying proper email security is imperative. While Google's own detection engine is already one of the best in the business, it's not perfect, and it is advisable to plug in additional email security gateways that detect malicious emails. Some very good options here are Area 1 Security (now owned by Cloudflare), IronScales, or Avanan (now owned by Check Point). All three that I mentioned plug right into the Gmail API, so these solutions are easy to set up and don't require complicated email re-routing.
3.) User awareness training. This is absolutely key and paramount. Get your users trained on security issues. There are quite a lot of professional tools available for that which run your users through easy-to-digest videos, puzzles, and questionnaires to help them be suspicious enough or even detect phishing themselves. Most of these providers also offer phishing simulations, which train and assess your users. Some notable vendors here are (again) IronScales, Knowbe4, Riot (very unique and special) or SoSafe.
4.) If you want to put some cherry on the top, get a security service edge solution that routes your entire internet traffic through a cloud-based security stack of a specialized security vendor like Zscaler, CloudflareOne, Cisco Umbrella and similar.
Unfortunately, there is no 100% security, but the closest you can get is through a multi-layered security stack that takes a bite at the apple from multiple different angles. Meaning, deploying multiple different point solutions, that as a whole, increase your security posture.
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