✔ 最佳答案
Well, I would tell him the truth that you are jealous! She very well may like him a bit, but he may like her a bit too if he got defensive when you asked him not to hang out with her. I would talk to him about it, and let him know how you actually feel and why you'd rather him not spend time alone with her. I hang out with friends that are girls every now and again alone, but I always let my girlfriend know what I'm doing and where I am, and if possible will hang out with them in a group as well.
There may be reason for concern, but the only way to really know is to talk to him about it. Don't go all surveillance on him, because you have to trust he doesn't want to do anything and won't with her. If you don't trust him, you may want to re-evaluate where your relationship is going, as trust is necessary!
Just how I see it, I don't know either party so can't say for sure ;) Good luck!