A friend of mine has been told to avoid sex in the third trimester of pregnancy. Is this true? Will it 'hurt' the baby?

If you look at the anatomy of uterus and vagina, male's penis is impossible to reach baby. Thus, it is perfectly fine to have sex in 3rd trimester if there is no complication. If your friend is worried it will trigger labor/contraction, you can suggest her to have sex after 37 weeks.