Can my girlfriend become pregnant from precum?...

can pregnancy happen in your case ?
its highly unlikely pregnancy to happen because usually precum dont have sperms , but if its happen to contain sperm , they amount of sperm is quite few and hardly to reach to ovum , despite the fact that there may not be ovulation during your sexaul intercourse timing 
we do we suggest :
- practice healthy sexual intercourse with using reliable way to avoid pregnancy 
- in this month , we suggest to do pregnancy test just to make sure 
How to know ovulation days and the safe days in period :
1. ovulation time or the time ovum exist are the first 4 days of the last 16 days of the period , for example ;if your period is 28 days , and  if your first day of period was 1st/9 , your ovulation time is 12th-16th /9 
2. the ovum last 48 hours in the uterus and can get fertilized any time during this period of time 
3. male semen lasts up to 5 days in female uterus and will be able to fertilize during this period of time , but the earlier the better 
suitable ways to avoid pregnancy :
- Ocp