Definitely forgive him. If you don't forgive there are terrible consequences. For one, you will not be forgiven by our Father, Matt. 6:14-15. Second, your prayers will not be answered, Ps. 66:18. Third, you will be tormented, Matt. 18:34.
If you have problems forgiving, welcome to the club. I think forgiveness is hard because we are so complex. We forgive on one level, but then another memory confronts us, and we are angry all over again. Just as an onion has many layers, so do we. We have to forgive completely--on all these levels.
So the best way to forgive, I've found, is to consider that when God forgave us, He gave us grace to forgive in turn. That is why Col. 3:13 says just as God forgave us, we must forgive others. Out of the abundant grace God gave us when He forgave us, take just a handful--and choose to forgive. Say it out loud, "I choose to forgive." Then give your situation to God, Ps. 55:22. Finally, ask your Father in heaven to take the hurt. He will.
This may take a while, as I said, we are complex. But every time those thoughts come up, go through those three steps again--choose to forgive, give the situation to God, and ask Him to take the pain. Eventually you will have forgiven--you'll be able to think about the situation without become upset again. It may take all 490 times Jesus talked about, but forgive and keep on forgiving anyway.
Jesus also counseled us to pray blessings. For a blessing you can pray on anyone, see Acts 3:26--that God would turn them away from iniquity (lawlessness).
As for being friends....I would be very, very careful. We are called to forgive, but we are not called to go back and be in an abusive situation again. Even Jesus went to the wilderness when He knew the Pharisees planned to kill Him.
Trust may be given freely the first time, but once it's shattered, it must be earned. It must be rebuilt, toothpick by toothpick, shard by shard. We must forgive, yes, but there is no reason to to back into abuse, or to trust so freely again.
