I agree with Belle. A long time friend either male of female would not be a concern. When that turns to flirting or emotional ties that should be reserved for your partner then you should probably distance yourself. These feeling may or may not be inappropriate but I'm sure you partner would not appreciate it. Put yourself in his spot and try to picture how you would feel.

The fact that you have posted this question tells me you already feel regret.

Wishing you the very best