Lawyers do not make decisions (or at least, should not). We exist to assess risk, analyze liabilities, advise clients, and hopefully, present options and strategies to minimize risk while still accomplishing a given goal.
Any smart lawyer in this case knows part of him still being hirable is going to be not having his entire reputation destroyed. Depending on the accusations, "say nothing" may not be a necessary or even valid strategy. Additionally, the client is the one paying. If they strongly feel they want to defend themselves, your job as a lawyer is to try to make sure it does not legally jeopardize their case (or warn them if it will and they want to do it anyway).
In any case, note that civil cases in most countries are strongly pushed to settlements. This is one of the reasons for broad discovery in most places.
All of this said, lawsuits against employees like this are pretty darn rare. Having been involved in a number of these situations on the other side, usually they are just bluster, basically a warning that "if he works on something competitive, we will come after you", or a warning shot to stop poaching. An actual lawsuit is usually reserved for exec and above, which i don't think he was (Sorry if i'm misunderstanding the level/ladder distinctions Opera uses :P)
Lawsuits like this tend to only result when someone has really pissed someone else off, or plans on doing something one side sees as really damaging.
Again, the above is my experience in the US, Norway may be different.
Outside of the actual legal realm, there is always a hearts and minds war. People stop paying attention to this stuff very quickly. Whatever gets said in the first few days will be the narrative for a long time. After the first few hacker news posts, this will go away for a year or two, and then someone will win or lose. In the meantime, if he doesn't speak, everyone will think of him as "that guy that stole stuff from Opera". On the other hand, if he frames the narrative right, it becomes "Opera, that company that is going down the tubes because it has resorted to suing former consultants on baseless charges".
Since nobody really wants to go to court, or lose mindshare (In this case, it's very valuable to both sides for very different reasons), whoever frames the narrative best tends to extract the most favorable settlement terms, or at least, go into the suit with a better stance and ability to press claims harder.
In an actual trial of course, it won't make a lick of difference.
He means the obvious "strongly advise him not to do it", in effect stopping him saying anything.
You over-thought that response too much!
Edit: slippery pronoun
