I'm going to lie and try to hide a man from my best friend
Why would you need to hide him from your friend?
I am worried that If I tell her about him and my relationship and they meet, she will steal him from me.
The fact that you two are in a relationship will get out sooner or later. Your friend will feel betrayed and hurt that you didn't tell her something like that and may end the relationship as a result.
I'll just deny, deny, deny and make it bigger than it is.
I do not see what you get out of doing this.  If you and your friend have an honest relationship, they will support you instead of trying to sabotage the relationship.