Did the Bb previously play in tune with the same setup (mouthpiece, reed etc.)?

If so, it's either mechanical, or the player is not doing something correct with embouchure. Check teeth on top of mouthpiece, firm corners, flat bottom lip etc.

Double check that you're using the correct reed and mouthpiece. The Fobes Debut mouthpiece is a great price and works wonderfully. Use at least a size 3 if not 3.5 reed for this.

Also make sure the neck and mouthpiece are seated all the way in before checking pitch. Be sure to use a neck strap etc.

Good luck!