feel totally uncomfortable at first, but trust him,
If they are best friends with their ex, that means I knew it when we got involved... and I'd be fine with it.
Get to know the girl, and make sure I communicated any feelings of insecurity or uncomfortableness with my boyfriend. Honesty is key. :)
Everybody is everybody's ex-boy-girl-friend in a small town. My country is loaded with these scenarios.
Make your feelings/ thoughts known to him...Though personally I don't think it would be ideal to stay with him when his bf is his ex!