"pregnant foetus - erm, nope, OP, your friend has an IQ lower than her shoe size."
You can be of low intelligence and still know this cannot be true.
My DD, has moderate LD's, has a true low intelligence diagnosis, yet is knowledgable.
Use the expression, "dumb as fuck" or "hard of thinking", but it would be nice to leave off the insults around "low intelligence", it is more or less the same as saying "is she retarded?", or making Mental Illness/Autistic jokes.