I offer translations here, and I have been asked to translate adult websites before.
I got worried and asked the Fiverr CS staff, and they told me offering a translation for
an adult website is OK since I am not the one offering adult services, I’m only offering translations. I know my case is different from your friend’s situation, but asking CS would not hurt at all.
Thing is though, let’s say you contacted CS, they tell you it’s not a problem for your friend to create a gig, but even if your friend makes it clear in the gig description that she will NOT be offering nude photos, I can easily imagine all the dirty messages she will be getting from nasty buyers. I’m assuming your friend is an attractive lady, I’m not sure if your friend would want to go through the troublesome situation of blocking all of those messages.