Will other members get notified when an admin makes someone a group admin in WhatsApp?
On WhatsApp, anytime someone joins/leaves or there is a change of privileges, in the chat there is a green notification in the middle saying that X person made Y person an admin.

There is a notification for it, although if you don't want your friend to see it you might be able to get away with it by spamming a bunch of stuff afterwards, although I wouldn't necessarily call that foolproof.