Is it weird my Fiance(female) shops with a male co-worker, whom has a girlfirend?

No mate, I have a best friend and I love spending time wiv him because he's fun and he is interested in a girl and I am madly in love with someone but we are just mates I like been round him because its good to have sometime apart from your partner because then you don't get bored and you have stories to tell when you see each other again

Not good. It can easily lead to other things, especially, when people begin to have doubts about their partners. Women are better at having innocent relationships with men but men are not.

That is true but been your fiancé is like an advantage to a girl she wants to marry you! And if she wants that, she won't do anything with other guys, she obviously loves and you love her because you are concerned but I think she is just having some time with a friend