Maybe I can help a little bit.
First, can we presume your friend's backhoe is a 4 point and not a 3 point setup? Kubota has offered both types; a 3 point would be more likely compatible, if not as capable as a 4 point, subframe type.
The Kubota backhoes do not all have the same 4 point mounts across tractor models and series, so the odds are against an older backhoe matching the subframe mount designed for a newer tractor. You might get lucky, of course, but you probably shouldn't expect it will fit.
As Carl says, get your backhoe model number and go from there. Some cooperation from your dealer will help a lot, since he will have details concerning the proper OEM-new backhoe and subframe combination for the B3300SU, from which he should be able to itemize the specific subframe part numbers for you and match them against the subframe specs called for with that used backhoe you're looking at.
I'd not expect the exact part numbers to be the same, so it may get down to taking measurements to see if the mounting geometry is the same.