Well, the flip side is that the mom also doesn't have the qualification to diagnose mental illness, unless I somehow missed her being a psychiatrist in the news. So just some Jane Random saying that a guy is crazy, even if it's her son, you know, she may be right, or may be wrong. And as we've seen in the Britney Spears case (albeit in a different country), even a parent may just try to game the system for their own benefit. There's a reason why the Mental Health Act says you have to be assessed by a team of professionals, out of which at least two different doctors must agree that it's necessary.
Another flip side is that even the Mental Health Act only says you can be forced into a hospital if your own life or health are at risk, or there's a threat to other people. If you're just being an obnoxious prick online, no, that's not even remotely enough to qualify. Even if you agree that some guy has been posting some really crazy stuff (incel or otherwise,) as long as they're not saying that they're going to do something about it, no, you can't force them into a hospital.
I mean, if we were to start putting people in mental hospitals just because they're obnoxious online, there would go half the player base of Counter Strike and other competitive online shooters
Especially if we're talking section 2, which would be the case here, which means they're not even diagnosed with any mental illness yet, but they're an imminent enough risk to need to be put into a hospital FOR the assessment. Yeah, it better need some damn good reasons for that kind of decision, since you're potentially putting a perfectly sane person through that, just because some non-qualified other person said so.
But basically the short version is that it's not as simple as just sectioning everyone the instant someone else said they're crazy. You also have to consider the rights of the people who could be wrongly put through that. You can't design a system which can turn into "eat your veggies and clean your room, or I'm calling the mental hospital"