I have old M&K speakers and I like them, but my taste is kind of irrelevant. I can tell you that mine* measure very flat, and I find them to be very neutral and uncolored, with headphone-esque detail resolution. They like lots of juice, but fed properly have pleasing dynamic capabilities.
You could do worse than vintage M&K, but depending on their asking price, you may be able to do better. If they are minty and your friend offers a friendly price, they may be worth it.
Did you get to hear them at your friend's house? What did you think? That's all that really matters.
*I have MPS1510, which are basically a single woofer version of the S125 you're considering.