It could very well be for the same reason that foreign credit cards are not normally accepted in China at many places.  I was told (and I have no confirmation for this) that the reason they are not accepted here is because merchants will need to wait about 6 months to finally get their money.  And when they do, there a re a lot of fees discounted from the charge.  

Chinese credit cards work with UnionPay as their interlink mode.  And for those that may not know what interlink is, it is the electronic routing that banks use to communicate and verify credit card purchases.  But UnionPay is corespondent with other interlink routes used abroad, like Maestro, Vista, etc.  So, technically, there is no reason to deny the charge, or refuse the card, as long as there is credit available.  

But many times things are done just because it would be a lot of work and effort otherwise, and maybe a degree of risk.

You can use the Unionpay card to take out cash, and that's what I do when I'm in Europe.

As long as your friend isn't being a stereotyical mong and wanting to buy everything in sight for face, he can just pay with cash.

my ex Chinese gf had few cards she used in the U.K, not a credit card but bank of China she could use it nearly everywhere and just sign for it .

If it's a Visa/Master card from the Bank of China, ICBC, HSBS, Citic Bank, there should be no problem. I had some success with Union Pay but only at ATM's and the ATM stated it accepted Unionpay