Hey all, I don't feel right calling you silly,
Just get your girlfriend on drums to play a quarter note kick, and have the bass player follow along in a 1, 4, 5 progression. After they get that going, you come in with the rhythm guitar.

It's not as hard as it seems, you may be surprised. Since you and your girlfriend are just starting out, the bass player should be happy to assist you. It will be a learning experience either way, but don't forget to have fun too.

Try using a boom chick rhythm on your guitar, and then try it with alternating bass notes. You'll be playing along with the bass, so that's a pick, and a strum for everytime your girlfriend hits the kick drum.

Let us know how it goes, and enjoy yourself.

JD