first of all basement or backyard boxing is dangerous and should not be done >
some1 needs to say this

but anyway, is your your nephew being trained by a real trainer or some1 with lots of experience?
if that is the case and he trains like 4 times a week and the other guy ain't training, or training without some1 who has experience, then i would say 2 weeks and the guy gets KO'd in round2.

but on the other hand if your nephew is training without a trainer or some1 with lots of experience... than
it takes a little more time to get any decent result, IF there will be any. cuz he might just be learning bad habits, and if he is only sparring with some1 who isn't that good in boxing either than he wont be learning any thing usefull during that sparring session. at least not in a few weeks.

best sent him down to the gym for a few times to learn some basic boxing and you should go to, and then work from there....

hope this helps
and succes with ya nephew