Ok, depending on your definition of "safe" and "stable" vary on experience. I install and uninstall a bit of software on my windows laptop and my linux pc. Because of this, windows starts to crash after a while, it's processes begin to hang on shutdown, things randomly disappear and magically reappear on my task bar. I've grown used to these things. In Ubuntu, adding and removing programs on a regular basis don't seem to affect anything.
I've also grown used to having to run virus protection, spybot s&d, spyware blaster, and a handful of other utilities to keep windows safe. These make it feel secure while I'm able to do this on my own, the average person has quite a few issues that in my opinion as a field techie, leaves them open for quite a few problems.
I remove viruses for people, I remove spyware for people, I help people learn how to take care of their windows machines themselves. I constantly see people using IE that went to the wrong website, or opened up the link that came in their outlook express promising Bill Gates would give them a new computer if they replied, or the free penguin screenserver makes these funny popups come up now...
This makes me prejudiced because the only call I get concerning linux is usually the RHEL 3 server or FreeBSD 4.10 box that's been running a mail server in the back room of a business where the hardware failed after %n years and it needs to be installed on a new machine.
I love linux, I'm an advocate of free (speech and beer) software. I don't think linux is ready for mainstream, not even ubuntu, but I do see it gaining momentum. Blaming linux for not being easy to use or not being secure, or not being stable is the wrong path. If you want to talk about grandma building programs from tar.gz packages, start trying to tell grandma to compile her own C++ programs on windows and realize that both are an exercise in futility.
Linux may not be the right path for everyone, but the real reasons that it's not the right path has nothing to do with any of the reasons listed in the initial post. If security, stability, and ease of use are major concerns about linux, then he should probably research into linux a bit more. If he's concerned that the software he knows already won't work in linux and that he won't be able to understand the inner workings as easily as windows which he's learned over the last 10 years, these are indeed valid concerns.