Communication is key here. You may just be curious about your exs best friend, and maybe itll even go somewhere real if they happen to be interested as well. In any event, if the split was amicable, its always best practice to respect someones wishes, so if theyre okay with it, its totally acceptable.

Can I still be friends with my exs friends?

You can be friends with your exs friends but it will not be the smooth friendship you had with them before. Like they would pass on news about you to your ex they would also tell you all details of who your ex is seeing and all those romantic details.