I think @Adirondackwannabe has the best advice so far. Don’t try to ‘prevent’ vomiting unless a particular poison has been ingested for which vomiting is not recommended, such as lye or other strong base, or an acid. (If you suspect any kind of poison, then contact Poison Control immediately and take their advice.)
But if she’s just ill with the flu, hung over, pregnant with morning sickness, ate something iffy from the fridge, etc., then all that’s required is to make her as comfortable as possible, give her fluids if she wants them, and monitor hydration. “A day of vomiting” for an otherwise healthy person, though it may make her feel like she’s dying (or wants to) won’t hurt her long term.
It also helps – a lot – to have a clean bath towel laid out on the bed (and maybe one on the floor, too) and a clean bucket or large bowl right there for when the run to the bathroom is just too much.