Yes, I would forgive someone who was deceitful against me. I believe that forgiveness is important and I wouldn't hold anything against them. However, this does not mean that I would fully trust that person again until they had proven themselves and showed that they could be honest and straight forward with me.
Why bother being friends with someone you can't trust. Not worth my time or effort. I know what loyal friends are. I think yes or no depends on if you yourself have been deceitful or not. Deceitful people are probably more likely to forgive because they've been deceitful themselves. Loyal people that are honest are probably less likely to forgive. Just my take!
No, I would not forgive someone who was deceitful to me, because forgiving someone only means that they got away with whatever they did to you. Holding a grudge is a good way to make sure that the person never does the same thing to you again. If you hold a grudge, you will remember to be mean to them the next time you see them.