Is your heli the same as your grandfathers heli?
If so, then you can substitute parts and see if the problem goes away.
Swap batteries, and see if the problem follows the battery. Meaning, put his battery in your heli, and see if your heli will still fly correctly. If so, then you have eliminated the battery as part of the problem. If your heli won't get higher than three feet, then you just identified the battery as the issue.
Look at his blades, and see if they are the same as yours. Maybe he put wrong blades on, or put the blades on incorrectly. I know it sounds silly, but I just got received a heli in the mail and part of it was put together backwards....so, don't assume everything is correct. You have to verify it.
On a coax, there are two sets of blades, commonly referred to A set and B set. Don't get them mixed up. He might have one A and one B interchanged...you need to verify what is what...
Lastly, IF they are both identical, you can see about binding his heli with your transmitter, and see what happens. If it flies right, then the problem is with his transmitter.
I've got you right where you want me.