My boyfriend lied to me Watch
If a bf wants to go out for drinks / social get together with friends, let him. You shouldn't be worrying about all the possible outcomes.
Ok, his ex / someone else's ex was also there. Like the bf knew. It's like hosting a house party for 30 of your friends, and 3 of them are people you used to date, because they tagged along as '+1' to another friend who originally got invited.
These things are not planned, they just happen.
Of course 'he never mentioned his ex going' because maybe he didn't even know she was coming!
Also, you say he didn't ring you back, even after saying he would. Well, after a few drinks, you forget what you said 30 mins ago (example) and tend to enjoy the company of friends rather than check your phone every minute.
Or his battery died / didn't have access to a charger / couldn't connect to the wifi in the visitor's home to whatsapp you. You don't know the circumstances.
If my phone was constantly buzzing with notifications, I'd put it on silent and check it when I got home.
I would talk to him about being honest in the relationship. You can't tell him to stop being in the same friendship group as her but you can let him know that you don't like that he lied about it.
At the end of the day, he has to be honest with you. If he can't keep that up then you'll have to reevaluate the relationship.