Trillian. Free in the Mac App store. Works just like the old Yahoo messenger, and better than iMessage.
Now you have done it. This is the same type of Mac user that will manage to infect themselves within minutes by:
1) They will Google "Trillian"
2) They will clicking on the first result link that will say something ridiculous like "Download Trriliian for Yaho Now!!!"
3) They will download a trojan from a flashing "Download Now" button
4) They will then try to install it but GateKeeper will stop them from infecting themselves.
5) They will then instead Google how to disable GateKeeper
5) They will then try to install the fake app and be stopped by the request for an Admin password.
6) They will enter the admin credentials and complete the installation of the Trojan
7) They will then try using the fake "Trriliian for Yaho" App to log into their Yahoo Messenger account (which is will steal their Yahoo credentials)
8) They will then Google why the App not working and will immediately be inundated with Adware, fake alerts, and the new JavaScript .node ransomeware in Safari
9) They will then call Apple Support and waste hours of a technician's time yelling about how insecure their new Mac was
10) The Apple technician will not be able to solve anything because the OP will not be able to prove proof of purchase, nor will they be able to receive email because their Yahoo account, Facebook, Gmail, and all others will have been compromised (because they used the same username and password for all of them and the credentials are now been sold on the dark web)
10) If the manage to get back online from their old PC, then the will come back here and bother us all again by complaining and raging about it was Apple's fault that they managed to infect themselves and that they will never buy a Mac again!