Everything is fair in business. Your friend has a right to ask you for whatever cut he thinks is reasonable. You have the right to negotiate that. It is simple.

Everything is fair in business. Your friend has a right to ask you for whatever cut he thinks is reasonable. You have the right to negotiate that. It is simple. But make sure you don't undercut/undermine your friend. I would just say "Hey 25% is a bit high and I would like to give you x% instead". You may now think that your friend is not really adding a lot of value by a simple introduction but if he had not introduced you to the client who probably are going with you due to the referral, you would make $0. That is how business works. Referral and Networks is the key and try not to burn bridges with anyone.