My girlfriend is radioactive after a PET scan, how harmful is this actually?

Not a big deal, I'm not worried but they'd told her to be careful about too much contact with people. Just wondering what the risk of it actually harming anyone else is? Is it just a precaution?

It is very short. Don't worry about it. It is not a danger to anyone else.

The half lives of the radioactive chemicals that they give you for a PET scan are really short. By the time she leaves the clinic, the radiation should be mostly out of her system. nothing to really worry about!

Damage from exposure to radioactive particles is a cumulative effect. The radioactivity in a PET scan, alone, does not cause problems. Problems from radiation exposure is likely to occur only when the sum of all previous exposures reaches a certain threshold. So radiation damage is prevented by minimizing exposure to radiation. PET scans produce a fair amount of radiation, so its a good idea to avoid the additional exposure if it isn't necessary.

when i had a pet scan i was told to avoid pregnant women and babies / children.

PET scans using radioactive fluorine in FDG would result in patients receiving exposures comparable to (or less than) those from other medical procedures, such as the taking of X-rays. Other scanning agents - for instance, 6-F-dopa or radioactive water - normally cause even less exposure.
For the patient, PET scans are very safe. You do have a radioactive injection, but this is a small amount and it goes away (decays) very quickly.

These days, some doctors tell PET scan patients that they should not have close contact with pregnant women, babies and young children for a few hours after their scan. If you are breast feeding, you have to express enough milk beforehand to get your baby through the first 6 hours after the scan. This isn't because there will be radiation in the milk. It is because the mother shouldn't be holding the baby closely during the time the radiation is in her body. Some doctors recommend you get someone else to feed the baby for 24 hours, although it is safe for you to express more milk for those feeds from 6 hours after the scan.