Sorry about my reply coming off so harsh,but when ever I hear someone saying start with a baby you will have a better chance of taming.I think you are setting them up for failure.Mangrove"s are all wild caught,they are very nervous and it can take a long time to get them to settle in(some never do)
Mangrove monitors cam grow to 5 feet,but most stay about 3 and half to 4 feet.They like to climb,swim,and dig s so you will need a very large cage 8x6x4 minimum.They also do a lot better when they can get above eye level.I would not recommend them for some one who has never owned a monitor,they do not make very good captives.If your friend has a tame one,that is very rear(I'm not saying its not possible)but don't count on yours being the same way.If you are really set on getting one then I would recommend you do a lot of homework,go on a few monitor forums and ask questions.
GREG