Mate, school is only the beginning of your education.
You don't come straight out of trade school expecting to be a brilliant tradesperson straight away. You do the course, then go and get practical experience, get to know the industry, read a lot, talk to a lot of people more experienced than you, do a few small courses to fill in gaps, and so on.
Some people know this. Some don't. Your mates are obviously the type who don't. I saw those types in PT school, not only will they not learn much after school, they didn't learn much during it, either. They were the ones who showed up late quite a lot, had to be chased for assignments, and so on.
From what you're saying, it seems that your two mates with the PT courses are not employed as PTs. There's probably a reason for that. If employed, they are probably the ones who spend a lot of time in the office checking email, or if on the gym floor, a lot of time catching up with their bros.
School's only the beginning of your education. A sensible person tries to improve their knowledge all the time, but recognises the limits of their knowledge. For example, I would not now wander down to PTC and start trying to tell one of Markos' 200kg deadlifters how to deadlift. But if someone came in who'd never deadlifted before, and if Markos asked me to, then yeah, I can show the newbie.
In regards to improving knowledge, one thing I've found is that sometimes something looks stupid, but when you look more closely, it's not so stupid. For example, when you see trainers leading people through exercises in the gym, those exercises might seem stupid and pointless to you. But if you actually asked why they were doing these things, it might turn out there's a reason for it.
For example, you might see me in a gym training with someone, and they're doing knee pushups and bodyweight squats. You snort in derision - surely I should be putting them under a bar? Well, it turns out that the person can only do 3-5 knee pushups, how much do you think they could bench? Maybe better to get them to about 20 full pushups first. And their squat, well if they can't do one well with no weight on their back, what's going to happen to them if we whack a bar on their back? So we improve their form first. And it turns out that person has half a dozen health issues which means that we want to take it slow and steady.
But given time, they will get under the bar. Remember that 90% of new gym members quit within 3 months. That means that at any time in the gym, most of the people are unfit beginners. So they need the basics first.
Yes, a lot of trainers are idiots. Lots cruise through the courses and get jobs anyway. But reasonably often, when something looks stupid, if you take the trouble to ask about it, it turns out not to be stupid after all.
Realising that is how you keep learning after school finishes, or even learn despite no schooling. Lots don't realise it, which is why you get people who assume all PTs are stupid, and at the same time why many PTs are stupid.
Saying that because many PTs are stupid there is no sense in having a professional train your girlfriend and do it yourself is like saying that because many restaurants are crap you should just cook for her yourself. Well, that depends on how good a cook you are. A lot of the time you'd be better off just finding a good restaurant. She deserves better than grilled chops and boiled vegies.
In any case, I didn't say it has to be a PT training them. I said a qualified or experienced person. This could be a PT, a strength and conditioning coach, someone who's been going to the gym for years and obviously knows their sht, whatever. And I said it should be someone not involved intimately with them. A parent-child relationship is alright for training, because it's partly teacher-student already. But brothers and spouses and so on, usually it's a bad idea. There are occasionally people who can handle it. but they're a minority.