This is very good question and I have faced the same issue with my parents-in-law and other family members of friends' families. Chocolate has given us an invaluable answer but to add context, I think the simplest rule to follow is "When in Rome..." or 「郷に入っては郷に従え」, and it does not make any difference whether you are in Kanto or Kansai：
In other words, listen carefully to see how the parents are addressed within the circumstances and follow suit. Typically in a friend's parents' house their role is that of お母さん and お父さん and that is how all visitors address them. If you visit with a group then wait to see how the group address them and follow suit. (One friend's parents, who I have known for over 20 years sometimes call each other Momma & Poppa even though it is long after their children have grown up. I think I should even follow suit here but somehow my western sensibility does not like this and I struggle to address them when they are together.)
When I first met my own parents-in-law I was also unsure how to address them but I noticed my wife called them お母さん＆お父さん and between themselves they used their given names. When we introduced our parents just before we got married this concern became minor and I quickly adapted to calling my parents-in-law お母さん＆お父さん when talking to them in Japanese (in front of my own parents) but using their own names talking English.
An easy way to see this naming in action is on Japanese TV when some celebrity is exploring a town and walks into the shop of a family business - they often show little hesitation about sayng お母さん＆お父さん and as suggested above, are really just addressing them by their function (in the same way that in the office one might refer to the boss as 課長 when talking about them and to their face.
(BTW: When in these situations I think it is good idea to look at the person in the eye as you address them for the first and be prepared to gauge their reaction and react accordingly.)