I have this lady and have been together for 5 years. I have been doubting her though I love her so much.
A week ago I happened to meet her friend/workmate and wanted to find out from her whether there is any thing going on since she is her best friend.
The lady told me something about going out with another friend but told me will disclose this after a week. She told me to wait for her call tomorrow so that she can tell me where my girlfriend will be and she is certain she will be with a guy.
I need your advise on how to handle this issue tomorrow if I happen to find out its true.
Thanks

First off if the lady is her friend, why is she doing this?
You have been dating a lady for 5 years and you doubt her? Why have you stayed with her for so long? If you doubt her, be a man and confront her, don't send one of her friends or co workers spying. That is dirty and it's none of that person's business.
If she is seeing someone else, leave her. What's there to question.