You will be able to play them at his house on his console but when you redeem your account onto his console it does not link the content to your account. So when you leave and redeem your account on your console the content would not be available to you.
Good attempt to cheat the system but you must think harder young one.
I'm going to presume that you are an upstanding member of society and not looking to fiddle the system... and you just want to use his super-duper DL speed instead of your rubbish DL.... (yes, I am most likely being overly generous there)
You can do the following:
You can go to your mates house, you can use your HD, recover your GT on his console, DL the maps to your HD, under you GT,
You can then go home, Recover your GT on your account and then play the maps on your console - although you may have to transfer the license to your box (not entirely clear on that bit though).
Your friend would not be able to use them on his box as he didn't BUY them.
Of course, you could DL the Map pack at your home, avoid all the hassle above and just watch some TV, walk the dog, eat dinner, do your home work, washing up, or whatever whilst it downloads...