Spotting while on birth control

By 'she forgot to take the pill after intercourse' do you mean she just forgot to take it at all that day? Because it doesn't matter when or if she has sex, she has to take it at the same time every day. (Just saying, because some guys really don't understand how birth control works, so just clarifying.) Anyway, if she did forget it completely that day (and didn't take it as soon as she remembered) she must have remembered the next day when she went to take that day's, and at that point she should have taken both. If that's what she did, then it's totally normal for her to have spotting (because of the large dose of hormones) and doesn't indicate pregnancy. I don't know why she wouldn't have taken both when she realized the mistake, but if she for some reason only took the next day's, then the spotting should be less. Normally one missed pill doesn't induce a period- usually it would take at least 2 or 3 full days without a pill.
Whatever she did or didn't do after the fact, it's still unlikely that she is pregnant if she was really good about taking the pills in all the days/weeks PRIOR to having sex. However, if she regularly misses or forgets her pills, it's really hard to give you an answer.
(PS You could read the drug information paper thing that should come with each pack of pills, it explains things like how the pill works and what to do if you forget and when you might or might not be pregnant, etc.)