I have to wonder what prompted you to re-pot your tree in mid-summer. Secondly, as alisios pointed out, your choice of soil was not appropriate. Keeping it inside 'near a shady window' is not really a good idea either, your friend apparently did OK with it outside. You may also be, especially considering the potting soil, keeping it too wet.
I think I would get it back out on the porch and follow the 'chopstick method' of watering as outlined in the general growing sticky.
It really would help to know the species. Can you ask your friend or post a photo?