If the friend is heating dry jars in an oven, melting butter, pouring it into the jars and closing the lids without further pressure canner processing- Home "canned" butter is not recommended by jar manufacturers or University Cooperative Extension programs in the US for several good reasons.

The reasons include safety issues with jars dry-heated in the oven rather than being sterilized, jars breaking during the oven heating, and the fact that what most folks seem to be talking about is not canning at all; neither water bath nor pressure canning. Butter is a low-acid food; thus needing pressure canner processing.

I grew up using both home canning and updated professional canning techniques. While many traditional methods that are no longer advised (the somewhat understandable "It hasn't killed me yet" school of thought) are still used in spite of our improved understanding of the risks of contaminated foods, the energy expenditure and health risks to family and friends who will eat the foods I preserve are not worth it to me in this case.

Butter freezes well, and (expensive) powdered butter as well as commercially canned butter is available. One can make ghee (clarified butter) to store for long periods under refrigeration. Those who still have a springhouse know they can keep butter and eggs quite a long time in the cool temperatures.