Birth can be incredibly stressful and makes the thought of sex very difficult, i had a very traumatic birth with my 7 year old son and i didnt have sex for nearly 3 years afterwards, just the thought of anyone near my lady bits was terrifying, maybe she has an underlying fear of partaking in sexual intercourse again.
There could be many reasons why she isnt wanting to restart a healthy sex life, often children make it quite hard, and a four year old is a very tiring thing!
The best thing your friend can do is to sit down, without kids about, and get talking, he may find she was deeply affected by the birth, or maybe that she is genuinely just too tired.