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This article may help: https://www.gardeningknowhow.com/plant-problems/weeds/controlling-sandbur-weeds.htm

You may find more articles by entering weeds into the search window at the beginning and end of the article.

Always read and follow package directions.

Often times, with competitive weed grasses in the lawn, it is just a matter of improving the conditions for the desirable grass. The desirable grass will naturally push out the weed grass.

Make sure you are watering correctly, fertilizing and removing thatch.

It is possible to restore a weedy garden to order. It won't be a one time fix though. Depending on how large the weeds have become, you might need to chop or cut back plants that can't be pulled by hand. I have seen people mow their garden before trying to till. Once the largest weeds have been "knocked back", the garden should be tilled or hoed to disrupt the roots of smaller weeds. Gardens should be weeded on a weekly basis. I've used sections from newspapers as a weed barrier successfully, and the newsprint degrades into the soil over the season. Also, IF, he isn't sowing seed, and planting only rooted plants, he could use a pre-emergent herbicide. According to package directions (very, very important). Pre-emergents work on the germinating roots. (And, I promise, if you use a pre-emergent, anything sown from seed won't come up, and if it does, it "won't be right". experience is the best instructor!)
This article has useful information: https://www.gardeningknowhow.com/edible/vegetables/vgen/garden-weeding.htm

You can till the weeds with the soil, but you may want to cover the garden area afterward. One of the best ways to keep the garden weed free is to place some type of weed barrier, such as plastic or fabric sheeting, over the area. This will help smother any remaining weed seeds and can be removed in spring just prior to planting.

I would try composting the manure/soil. The heat from composting will usually kill any weed seeds that may be in it. Here is more information: https://www.gardeningknowhow.com/composting/basics/composting-basics.htm, https://www.gardeningknowhow.com/composting/manures/horse-manure-compost.htm, https://www.gardeningknowhow.com/composting/manures/cow-manure-compost.htm, https://www.gardeningknowhow.com/composting/basics/heating-up-compost-pile.htm

This may be dependent on the type of weed (those with long tap roots will be more difficult to eradicate) and outdoor temps (generally need fairly warm temps to work well). You may also need to reapply these until they take effect. Boiling water will typically kill anything it comes into contact with, regardless. This article has some other suggestion if you haven't yet seen it: https://www.gardeningknowhow.com/special/organic/homemade-pet-friendly-weed-killer.htm