Is it common to call a waiter "朋友"?
I think 服务员 is just a little bit old, formal and sometimes serious way of calling waiters/waitresses from some people's prospective nowadays. But I don't think it is offensive if you use it, especially if you're a foreigner. I also use it several years ago and even now.

Alternative way maybe 服务生(mostly for male). I guess it's the way from Hong Kong or Taiwan but accepted in mainland. This sounds more modern in some cases.

朋友 is not widely used by local in such cases. Maybe because you're a foreigner, they would like to show more friendly. I think they can call you 先生（Mister）/女士（Mrs.）/小姐（Miss）. However, if this is the cafe/bar's feature or tradition or something like industry rule (though I don't know such rule), then it's OK.

Instead, you can just call waiters (even not only waiters, but any other male you don't know)  as 帅哥、小哥、小哥哥（if he is young）； 兄弟、哥们；叔叔、伯伯、大叔(if he is elder)；大爷（very old）...
The same, call any female you don't know as 美女、小姐姐、小姑娘（if young）;姐姐、姐们儿；大姐、大嫂、阿姨（if elder）; 大妈（very old）...

小哥哥、小姐姐 are just the popular saying recently, we don't even use them maybe 2 years ago. Things are always changing, you know.

Finally, it's simple and proper to say 你好 in order to get the waiter's attention.