Terrie.....I can understand your desire to maintain confidence and privacy for a grandchild. Naturally, knowing just a bit more of the situation, makes giving advice a bit more helpful.... But to respect your intentions, I'll do my best.
Frankly Terrie, a teen is a teen, whether your very own or "borrowed" from one of your adult children. I realize many things change from one generation to the next, but we're still talking about one child-one supervisory adult, who has been through the experience already. I am not sure you need to feel it's that much more difficult or different.
If you can look at it from that perspective, you may be able to relax a bit and be the parent you already know you can be. Spending quality time and having mature discussions (not talking down to him/her) and trying to remain upbeat & positive, are very vital things to focus upon. Teens seriously want (need) to be heard and know that their feelings are normal and that you validate them. You can also be comfortable being quite "matter-of-fact" when speaking about right & wrong, acceptable and unacceptable, moral and immoral ways of behaving and of looking at the world. Sharing some of your own experiences and incidents as a teenager always goes over well with them. If you can help them to see YOU as having been one of them....this gives them some comfort as well.
Keep your expectations of him/her realistic but on the "high" side and let them know you are just a reach away for assistance when things get tough. Stress their education and encourage their creativity and talents (even if they seem a bit silly to you now) Give him/her responsibilities in your home and hold them accountable.....praise/reward Or correct & teach when called for. Make it as easy on yourself as possible without seeming unattached.
I have faith you will do a fine job. That you have sought advice is a plus to begin with. This tells me where your heart and dedication are. So, Gram...you can do all that you feel is necessary and add some creative ideas to see how it works with this particular child. Keep your eyes and ears fully open, wear a smile and try not to panic. While this is universally a rough spot, it passes. The idea is to make it out alive and sane while the teen walks away with a strong sense of the world around him/her and enough confidence to challenge himself to be successful.
I wish you much luck and a very good outcome.