Pour on the feedback on how well he has been progressing as a player, how you see his skills developing, how much fun he is to play with and how much you love him as a buddy.
Then also start to work in the tactful assessment that someday you hope he'll be top billing, with you as his sideman trying to keep up.
And then go on that you are planning to diversify the band a bit for various venues, and that you sure hope to keep working with him at many of these.
Then ask him to listen to the harp dude you want, and if he thinks he can get that good, and how he will go about getting there, and whether he agrees that the other dude is ahead of him and more suited to some venues.
Talk about how that other harp dude raises YOUR level of playing to degrees you had not known you might aspire to reach.
Focus on the VENUE and the venue's expectations, not the "gig-worthiness" of the opportunities or the gig-worthiness of the two people. You do not play for each other. You play for audiences and serve their pleasure.
And then-- if you do all that and feel wrong about it, knock it off and go with your buddy. He might not be the most sparkling tooter, but he might be the easiest to work with the the lowest-maintencance bandmate to sign up solid. He might could use a little challenge to raise his game. :~) Are your expectations high enough?
And what does P-Vine say? You know she is always going to be right on matters of the heart. :~)