I would recommend just warning them and not treating the soil at all, unless they request a specific treatment when you ask, or unless they're in another state or you know your soil has particularly bad weeds, pests, pathogens or something.
You could pour boiling water on the soil to kill weeds and bugs, but it will also kill beneficial soil microbes, which may result in smaller cucurbits and give fungi an advantage if there's not a lot of sun on it. If you could do that and then add beneficial microbes, right away, that might be great. I've never tried adding beneficial microbes, unless you count yogurt in a container indoors, but that didn't end well. Mixing your sterilized soil with your friend's poor dirt could possibly colonize it with good microbes, but I don't know for sure.
There might be workable pesticides and herbicides you could use, but your friend might be into organic gardening. Again, I recommend asking your friend before you do anything.