I don't think what you're looking for exists. There has to be some long combination of fittings to get from one to the other, but the more interior shoulders and more fittings you have, the more issues you're likely to have with CO2 getting knocked out of solution causing foamy pours. I'm not sure why the hesitation to install MFL fittings into your friends lines. It won't change the functionality for him at all.
I'm trying to find some porn filmed by a dog wearing a GoPro. - SharonaZamboni