My girlfriend sleep score is 3/100 is it because she used my watch?

The sleep score is based on a number of factors that you see in the list. The low score comes from: Low sleep duration, no deep sleep, not low in stress, almost 1 hour awake time in first image.

I always advice people with low or wrong sleep duration to check the movements during sleep. The presentation of that is best in Garmin Connect Web: https://connect.garmin.com/modern/sleep/2021-07-20

Maybe she moves around a lot during sleep which makes it hard for the watch to calculate sleep correctly or her HRV is low.

This is her movement for the poor sleep scores.

This is mine for high sleep score

Looks pretty similar. 

So me having a baseline of data doesn't screw up the sleep score for her? 

Yes, if the watch is using your typical HR, stress levels, etc as a baseline to calculate sleep, it is likely to produce a misleading sleep result if someone else wears it.

An overnight low HR of 10 above your normal resting HR would normally indicate something going on.

If you are going to share the watch, set your GF up with her own Garmin Connect account, remove the watch from your account, and assign it to hers, delete data from the watch, then sync.