First I would just be honest with him that your curious about the circustances. Don't get mad jelousy doen't get you anywhere. Don't change friends you can't go through life expecting everyone to be blind to the fact that you have an attractive spouse. Self esteem is the key here. Be comfortable with yourself. You have to have trust in him. 

1. Trust your instinct. Is she flirting Yes or No

2. Do you trust him . If yes  so what if she is flirting He is yours and you take it as a compliment that you have him

3. Don't be Nieve.     If she is out of line  continuesly,  address it to him first that you are uncomfortable. Stand your ground and don't be a push over. Stand up for yourself .toward her - Don't cause a scene, that puts fuel to her fire , but express yourself  Directly to her- be blunt to the point but NOT rude . 

4. Always love yourself