Let me use a few analogies:
You are in a store looking through the magazine rack. You come across a magazine that has a blank cover. You open it up and discover that it’s pornographic. You close the magazine, place it back on the rack, and don’t give it a second thought.
That would not be a sinful act. Even though you deliberately opened the magazine, you did not initially know that it was pornographic nor did you continue to look at it.
You are in the same store looking through the same magazine rack. You come across a magazine that has a blank cover. You open it up only to discover that it’s pornographic. You continue to look through the magazine - in fact, you spend a considerable amount of time looking at all the different pictures.
That would be a sinful act of a grave (perhaps even mortal) nature, because even though you initially didn’t know that the magazine was pornographic, once you discovered that it was, you deliberately continued to look at the pictures for an extended period of time.
You are back in the same store; same magazine rack. You come across a magazine that has a picture of a scantily clad woman on the cover with the words “Sizzling Hot Naked Chicks” written beneath it. You grab the magazine (like there was no tomorrow), and start going through all the pages (all the while salivating like a mad man). Whether or not you choose to do so for a short time or a long time is irrelevant.
If you even have to think about whether or not you committed a grave/mortal sin under THOSE circumstances, you definitely need to “get right with God”.
Insofar as your friend is concerned… since you already know that the pictures he will show you will most likely be of a pornographic nature, you have a responsibility to ask him to stop. If he is really your friend, he will respect your wishes and do as you ask. Under the circumstances, however, it’s all pretty much on you at this point. God holds us accountable for the company we keep.
If he is persistent and refuses to comply, he isn’t being a friend at all.
My mother is a devout Catholic, and is friends with a Presbyterian family that she has known throughout her lifetime. They have ALWAYS had the utmost respect for my mother’s views, religious beliefs and traditions. That’s what real friends do.