*If someone is falling, rather than try to keep them upright, get them as safely as you can to the floor. Ideally, lean them against yourself and slide them down. It's a useful thing to know if you're taking care of someone who's old or ill enough to be unsteady: https://www.youtube.com/watch?v=o6PRQ0CRn5w
Now you've done a good deed by sharing that :-) I've been in that situation a couple of times with friends in a public place, and never knew quite what to do.
Yep, that's why I shared it.
Once upon a time, I was taking care of a loved one who was ill. I wasn't right there when he started to seize, but he had the presence of mind to speak up just as it started, and I got there quickly enough. I did not know what I was doing, but I managed to keep him from hitting anything too hard on the way down, which was all that mattered. The paramedics took it from there. After that, it occurred to me to look up what I should have done, which fortunately turned out to be a more controlled version of what I did.
By the way, if someone is feeling dizzy or faint but is still at least partly in control, stand behind them with your hands near their hips, and help them to take a seat. The ground or floor is a safe place for them to wait while the next steps happen. That is, call emergency services and keep the person safe, unless you know otherwise. (You might know otherwise if you know from last time that the person is diabetic and just needs to drink some fruit juice and sit still for a while. Still call for help if you have any doubts, of course.)
There are also videos on various techniques for helping someone up safely from chairs, beds, and floors, so you don't injure yourself or them.
It's not exactly first aid, but it seems like this kind of thing should be more widely taught, especially given how many people at some point end up being the home caretaker for an aging or ill relative.