Hi Larsenio,
According to this
page, it would not be safe to connect it to a recent Pentax camera.
However, as you said, you have used it on your K10D without any trouble so I would think it is safe to use it to your wife's K-r...
There seems to be some versions of this flash with lower trigger voltage, it looks like you're lucky...
I have read somewhere that the "safe" trigger voltage for Pentax DSLR's is 12V. I can't find where I read that, so you might not want to take my word on it!
Maybe somebody more qualified can give you more info...
François