First of all, you sound like a real dreamboat. I mean that. You are being patient and kind and helping your gal build her game. Few guys would do that. Props to you.
One thing you should keep in mind is that women were Born To Lob. We come right of the womb knowing how to lob. So one thing you can do is stand at the service line or closer and have your gal feed you lobs that would bounce on the baseline. Practice control on your overhead/swinging volley right back to her. Teach her to slice or block that next ball as a lob, and so forth.
If she is 3.0 and can own a consistent lob, she will beat . . . well, she will beat *everybody* at 3.0. Few at 3.0 have an overhead. Heck, I am walking proof that even 4.0 women can lose to a good lobber.
-- Random Error Generator, Version 4.0
-- Master Moonballer