There are two problems here. One is the size difference, and the second is the sex difference. Since the house "belongs" to the Lhasa, he probably assumed he would be top dog. The new female would not allow that. Since she is larger and stronger, and also because she is female, she wants to control the "nest." She will probably not hurt him on purpose, but she will probably toss him on his back and hold him down. If he does not struggle, the fight will be over, although she might toss him around from time to time just to see how he responds.
If he does struggle, or he bites her, the problem will escalate. She will feel the need to toss him down almost everytime she sees him. If you interfere and scold her, it will most likely make the situation worse. She might wait until you are not around and then actually hurt him.
This is not a simple problem. When you brought the young female into the house, you changed the dynamic. If these were my dogs, I would immediately enroll them in obedience classes so that they would be more responsive to me and not take notice of each other as much. That is no guarantee. I do not know where you are located, but you can also ask your vet for a reference to an animal behaviorist that is closest to where you live. If the Lhasa mix is frail, that is your best option.