Then do that. It seems like you have a double reason for trying it at least once: For her and for You.
Let her arrange it. Have her lead him in right from the shower. Do it in way that's comfortable for you, either laying in a bed with you on one side of the bull and her on the other side ... or, you sit in a chair, and have him stand in front of you. Touch his cock, feel how warm it is. Let her get him hard. Touch his cock with the tip of your tongue. Follow your wife's lead, let her show you how to lick it up and down the shaft. Let her put the head of his cock in your mouth. Let her guide him back and forth in your mouth, holding his cock at a certain point to prevent him from going too deep.
I'm sure your wife would be overjoyed to help you suck her lover's cock for the first time.