Some of my closest friends are Koreans, and I have always taken information about Japanese from Koreans with a grain of salt and vice versa. Many of my Korean friends have claimed that at some point in the history of Japan, rape was either legally or socially permissible or even encouraged at some point in Japanese history, and that even the Emperor at that time did not care about any punishment for rapists and encouraged rape.
This seems absolutely ridiculous, but I kid you not that they claimed that a woman could simply go to the barn/outhouse and men would come and rape them and they had to simply take it without any sort of retribution from either the law or society. I thought they were talking about ancient times or something, but it was something like within the past 300 years (forgot exactly which era or Emperor it was at the time). They also claimed that it was suggested for women to wear the kimono, because it is a garment that's front is easy to open and rape women. One of them even claimed that the kimono was invented for the sole purpose that it is easier to rape women in.
It sounded ridiculous to me, but my Korean friends as well as their acquaintances were treating it like common knowledge. Due to some reasons, I have extremely strong views against rape (more so than the average sane person, I guess), so it was an uncomfortable topic to discuss and research, but I really ended up with nothing substantial about it from the internet. But since it's bugging me right now, I thought I'd ask here.
While I can not speak for all of Japanese history, Your question seems to be asking about the relationship between rape and Japan, so I will answer with what I do know. My knowledge of history is more in the area of China, but occasionally there is some overlap. I can not prove or disprove the association with kimono and rape, but there were other times in Japanese history where rape was involved.
One such time would be during World War 2. This topic is one of the most-talked about when it comes to women during the war. During Japan's occupation of China and Korea, the army employed the concept called "comfort women." When your friends mentioned the "barn/outhouse and woman would come and rape them" I am assuming they might be talking about this.
Comfort women were Chinese and Korean women that were held against their will im order to pleasure Japanese soliders. Some of these women were promised compensation, while others were simply taken from their homes. The most brutal depiction I've seen of comfort women were girls quite literally lined up laying next to each other in stalls, men would go in the stalls, pleasure themselves, and leave. There were women who died from this. So as far as your question of rape being socially permissible or encouraged, I would reccomend looking at the comfort women and saying that yes, rape was encouraged and allowed against Chinese and Korean women during this wartime period.
As far as modern times are concerned, the last time I was involved in anything regarding comfort women the japanese government still refuses to acknowledge that what happened was rape. Most of the time Japan's government likes to claim that everything happened consensually, comfort women were taken with their permission, they were paid prostitutes, etc. this is similar to a lot of other things that Japan did in World War 2. Accountability is not something Japan likes.
I think its very smart of you to take your Korean friend's words with a grain of salt. There is a lot of bad blood between Japan and Korea (because of World War 2. Comfort women was not the only thing Japan did at that time to put them on bad terms with Koreans).
If anyone else has anything to add regarding this topic, I will be following it. I can only give information regarding what I know (which is just overlapped with Chinese History.)