We rebuilt a bed to which this sort of thing happened, though it was a twin and the frame was only a bit cracked. But super long L-brackets down the length of the frame worked nicely to both brace the frame and serve as a place for the slat-ladder to rest.

Actually, now that I think about it, we rebuilt at least two.

They were trundles, one (or more, I don't remember) from IKEA which would have been about a year old and one a bed I had when I was a kid (it cracked when we lived in Cambridge.) The trundle clearly didn't do much to support the slats (which is good because otherwise you'd never be able to draw it out and use it) but it did mean we only fell a couple of inches instead of all the way to the floor and this may have been what saved the frame from worse damage. One of the times involved wrestling, which I only remember because I would tell people this and they'd all go "wrestling, huh, wink wink nudge nudge" and, yes, wrestling. The other time people were just sitting on the bed quietly watching TV. Thrud complained because she said beds should be designed for two people to be doing something somewhat athletic on them. Possibly despite the evidence of college students in dorm rooms, no one expects people to be doing anything athletic on a twin.