Does a plugin exist that lets you have instant message between users?

Some of my clients want an integrated organisation chat system into their intranet, that pretty much has all the features of Slack. I’ve tried suggesting they just use Slack (duh), but they want something stand alone. I can’t find anything and I was just wondering if anyone had experience with anything that worked for them. If not I’ll just tell them to use Slack and not be dummies about it.

I’ve looked at BBpress as the closest reliable alternative, but they’ve given it a pass.