Wow....I work for the Fire Department and even THEY can't tell me who to hang out with, or what I can do outside of work! they can't even tell me I can't have a second job!
obviously there is common sense involved here. nothing illegal, and I can't go wearing my uniform t-shirt to the bar and get stupid!

the way I look at it is this. if you aren't going to pay me 24-7 then you can't tell me what to do outside of work!

are the two involved in a direct supervisor situation? ( is he/she YOUR boss?)
I could see the "no fraterinzation"(sp) rule being sited... but since that rule is usually under the sexual harrassment section of an SOP book and you aren't "hooking up" then I don't see what the problem is.
I'd consult the HR person, and if you have a union talk to your rep. and after that you don't get a satisfactory answer as to WHY and you feel strongly enough talk to a lawyer. NO company should have control over what you do on your own time.

Scott-work to live not live to work-Ellis