Thanks for posting your query.
is supposed to be taken within 72 hrs of an unprotected sexual intercourse to prevent conception. It is ineffective if taken on a later date.
In your case, your girlfriend has taken the pill on Friday after the intercourse on Monday which may not be of much use.
The irregularity in her periods is expected after taking Ipill. It should regularize with the next few cycles.
If you still want things to be safer, you could get her examined by a Gynecologist.
Hope this helps.