Seems like everything nowadays is made to work perfect with brand new stuff but falls apart with the slightest wear. Same thing with my truck's tailgate latch - worked great on the lot but couldn't handle a few years of dirt and use. Your buddy probably just hit the same wall we all do when trying to fix something that's already been through the wringer.
The 3-in-1 connector I've got has held up fine on all sorts of hoses, but I wonder if your friend actually checked inside the connector for any debris before using it. I've found that a tiny bit of grit or an old washer can cause a leak that's easy to miss. Did he take it apart and clean it out thoroughly? That might be the real issue rather than the connector itself.