That I do! 99.9% of the time I'm carrying IWB. I'm about her size, too (5'9" & 120), and probably like to dress about the same. The Shield I carry is only a little bigger than the G42. I use an Alien Gear holster most of the time, a 3Speed sometimes, though I find it's a bit too bulky with a small frame. While I love the 3Speed, I wish I had this instead:
Link Removed If only I had the extra $80 I'd get it in a heartbeat. She wouldn't need a belt for that.
As for clothes, I wear my normal stuff but add a cover garment. No going up in sizes for pants, either. Denim is stretchy nowadays. Just add a light vest, or some of those yoga cover-ups. What works best is a plain button-up shirt left undone over a tank top. Anything that hangs loose from the shoulders works perfect for carry at 4-5 o'clock. This is the belt I use:
Frequent Flyer Belt Normal leather gun belts are too thick and more difficult to conceal than the gun! That one's thin but sturdy... Someone on the forum suggested it back when I was looking.
And definitely look up faliaphotography on youtube... She's almost exactly your fiance's size.