Point out to him - if he does not already know this - that the Pope gives the references in the Syllabus to those of that Pope’s letters from which the condemned propositions are taken.
They each address a specific problem - and the circumstances which make a condemnation of an idea in (say) Hungary which is then condemned by a Papal letter in (say) 1853, might be inappropriate in the UK in 2006: the condemned idea might be perfectly harmless, in some contexts, or even be Papal teaching.
IOW - look at the meaning of the condemned proposition in their historical contexts. That will show
why there is a condemnation, &
what is being condemned. Words alone are not everything - the ideas they express, in their historical circumstances, are what matter.
Otherwise Jesus would be convicted of fraud by Christians every time they read in Isaiah: “I am the Lord, & beside Me there is no other”. What a final proof of the falsehood of Christ that is - if only words by themselves, and not history or their context, are considered
Newman dealt with this ill-informed approach to the Syllabus well over a century ago, here:
newmanreader.org/works/anglicans/volume2/gladstone/section7.html
It is bad theological method, & bad reasoning, to take a single sentence from a letter, & to treat it is a universal and exceptionless & timeless judgement on everything that may in one’s own time & country look like it. The USA in 2000 is not France (say) in 1850. One has to know why the original condemned proposition was condemned. This is basic (un)common sense.
Hope that helps