without following that person try insteye.com. There is no such animal as online tool that will allow you to breach the standard privacy IG settings.
There is actually a way to view private Instagram photos without following that person with your account
If you don't want to let them know that you are following them, but you still want them to add you so you can see their private pictures on instagram, then here's the steps to follow:
1. Go on Twitter or Facebook, and find the person.
2. Find the person they interact with the most that isn't on Instagram already.
3. Create a fake profile of that person on Instagram using photos and ideas they have mentioned on Twitter or Facebook (or any other site like Pinterest, Youtube, or even Flickr).
4. Add the person who has the private profile you want to access.
They will add you to their private Instagram profile in a few hours, and it will take them at least a few days to work out that you've managed to go under the radar and hack their Instagram friend's list. Sometimes, they don't even notice!
4 people from my school added me on Instagram thinking i was someone else, and I can STILL see their private profiles even a year later :)
There are other ways to hack Instagram if these method sounds to compliated though.
Check out this guys answer instead: http://internet.blurtit.com/3942450/how-to-view-private-instagram-profiles-hack