When doing the cardiac cath procedure (angiogram) through the radial artery, an incision is made in the artery near the wrist, and a catheter is then inserted into the artery.
The catheter is then advanced slowly up into the arm artery until it enters the heart vessels. There, the catheter can inject dye into the vessels so the doctors can see if there are blockages in the heart vessel itself. If a blockage is seen, then a stent is inserted in the vessel to open it up.
Bruising and bleeding are unfortunately complications of any stenting procedure, especially if someone was on blood thinners (like coumadin, aspirin, aggrenox, or plavix) prior to the procedure. It sounds like you are doing the right things by watching your dad's bruise (hematoma) closely.
Now, your question is about the risk of internal bleeding. The highest risk for internal bleeding is right after the procedure, which is why you are kept at the hospital for a observation after the procedure is done. The nurses monitor your vital signs as well as the procedure site for several hours to make sure that everything is stable. If there is a change, then the cardiologist would be called to come and assess the potential for possible bleeding.
Once you are discharged home, the risk for bleeding is much less. However, it is possible for him to bleed from his hematoma. Signs that this would be happening include:
1) sudden increasing pain at the hematoma site
2) a noticeable increase in size of the hematoma.
The pain and swelling you mention can be from the current hematoma and bruising, but it should lessen over time. If the pain suddenly becomes worse, you should call his cardiologist and let them know. There is also a possibility that the hematoma could develop an infection. Signs of this would be that the hematoma became very painful to touch and felt "hot". One thing I would suggest for you to do would be to use a black marker (Sharpies are good) to mark the areas of bruising. Draw lines around his arm, near his elbow bruising, and actually mark the circle of the hematoma. This way, if it should become larger or more bruising should occur, you will know right away.
Although it will take some time, the hematoma and bruise should fade, and hopefully, he will do fine. Good luck!