I'd say it's better keep the body oxygenated and unobstructed than doing nothing at all. You will hear stories about patients coming into hospitals with CPR related rib fractures, but is the person dead? No. Improper technique? Yes. The victim has a better chance of surviving if someone acts quickly enough to buy enough time for professional medical care and AED to arrive.
i like this line of thinking, when i was first taught cpr in high school, they told us the breathing was the important part, and even discouraged the pumping because of injuries. but with the thoughts now that the circulation is more important... perfect reasoning for me to never have to put my lips to a strangers unless i'm kissing them for fun. i love progress.

and if i ever need cpr... i hope someone like you two is there and has a mask haha.