Could you possibly put a pic up or get your friend to - if, as I suspect, it is one of those Lidl type 'bonsai' then nothing you do is going to keep it alive!

I agree with Moon growe, A picture would help. I have several bonsai and the biggest mistake people make is thinking they cane grow them indoors, but they are trees and will not live indoors unless it is a tree specifically as an indoor tree and these are very few. Bonsai need to feel the seasons outdoors. I suggest you buy a book on bonsai, this will tell you all you need to know. I suggest the book the Bonsai pocket encyclopedia by Harry Tomblinson. You may have to order it but its a great little book.

Yes, we definitely need to know what species of tree it is, since the requirements vary greatly with the species. If it lacks a label, a picture would help us to identify it.

Chino, wise words. So many little trees succumb to being left indoors on the advice of garden centre staff. Mine all sit outside getting frozen solid in winter, and gasping for water in summer! Still with me, though

Thank you all so much for your answers. I thought after I posted it that I really should have given more info. I will ask my friend when I see him for the name of it. I always thought you could grow them indoors so now, thanks to you all, I know differently.
The roots of my friends are growing out through the bottom and he would like to know if he should trim them.
He will look for a book. Nice talking to you all.
Not feeling too bad now Meadowland.

It will be OK to trim the roots from the bottom of the pot.
It very much sounds like the tree is pot bound and needs attention, but best to wait till he has a book which will tell him what to do and when to do it. But do tell him to make sure the tree is watered every day and if it is a hot day it will need watering twice a day, morning and evening.

Thanks again for the info. I have passed on all your tips so hopefully he will be successful.