I can't give advice for Facebook ads. I've never ran one. Have you ran AMS or BookBub?
I've done plenty of AMS and BookBub ads. My fair ads range around 15-20% sales per dollar. These are average for me. So for most of our books averaging $2.99 to $4.99, that equates to about one sale every $20, which matches exactly with your friend's experience. For a series or boxset, this can work well. Like your friend said, for a single book, it doesn't.
So you have to experiment to get a better paying ad. Some better ads will approach 1:1. In other words, you put a dollar in and you get a dollar back. Or you pay $20 and get $20 in sales. But it takes a lot of trial and error to get those--one would argue, like with betting, you have to lose a lot of money in the front end to eventually win. Of course, the optimum ad yields more sales than you pay. I have one ad that's been very successful out of many many failures. It yields around 1:1.