[SWHouston]

Rachael,

First of all, let me empathize with your Dad, I would have just gone nuts, given that I'm not able to keep up with one of mine, if it got loose.

Instead of buying another Harness, you might try this...
Buy a simple soft Collar, which you can adjust to fit
around your Dog's waist, just before it's hips.

Run your existing Leash through it, connect it to the existing
harness, THEN, tie a knot in the leash, where it runs through
the D-Ring of the (extra) strap.

I know that the picture doesn't show your exact harness, but
your existing stuff would be in red, and the new strap/collar
would be in black.