She could try elevating her hips after sex. It is totally normal for the sperm to come out of the woman's vagina after sex. The vagina is mostly a sheath, it only has a tiny opening at the far end by which any sperm can swim into the uterus. In short, sperm always comes out. But if she would like a little pool of it to stay longer near the small opening (into the uterus at the other end of the vagina) to increase the chances that some will enter uterus, she could try putting her hips up higher than her body with pillows for a while after sex. Some women even try a headstand.
Have sex every second or third day in the time from the end of her monthly period to the beginning of the next period. Not every day, though, since that would have a negative effect on your sperm count.
If these things don't help, see a doctor and get your sperm count checked, and also be sure you don't have a varicocele, which might heat the area a little bit and have a negative effect on your sperm count.