Is there a range of time (ie 7-8 months) before it's considered safe
to switch companies? Was I correct in asking her to at least stay for
a year (in fact, maybe more)?
There's no magic time range. It's less about the amount of time, and more about the pattern of behavior.
Few companies want to hire people who won't be around for the long haul (The exceptions might be minimum wage jobs, or jobs that hire temporary or transient workers. I suspect even they would like to hire people who could be expected to stay around for a few years, if they could find them.)
When I hire someone, I put a lot of time, effort, and money into finding them, paying them, training them, and integrating them into our workplace. Then only way that works out for my company, is if that person sticks around. I value a stable work team a lot.
Your friend is showing that she can be expected to leave after 2 months. There's no way I would hire someone like that.
But if your friend had been in 3 jobs for 1 year each, I'd still be extremely unlikely to hire her (particularly, if the reason she left each job was because she "couldn't stand it there"). Her work pattern would show that she would leave after 1 year - that's not what I'm looking for.
The pattern your friend has established shows that either she doesn't know what she wants in a job, doesn't know how to find a job that offers what she wants, or simply doesn't care to be at a job for any length of time. For many employers, that is a bad sign.
If she were my friend, I'd advise her to think long and hard about the kind of job that would satisfy her for a long time. And I'd help her learn how to interview better, so that she could determine if the company will meet her needs before she accepts a position.
Having her grit her teeth in a job she hates just to make it to 1 year, probably isn't going to change her behavior/hiring pattern, and thus probably won't help much.