Just be open and honest with him, you’ve had a poke about and discovered a Web-discounter, plus Top cash back deal that saves £150 over what he’s offering, but you’re aware you’ll not be getting much aftersales service or technical support, could he meet the discounted price halfway?
Or perhaps simply pay (with money, not biscuits) for an hour of his workshops time to get the bike setup properly for you?
friendship shouldn’t be about money, being mates shouldn’t be contingent on you throwing cash at his business, if he’s an actual mate He’ll understand, £150 is still a significant sum to some people…