You're both right, though it's a bit weird of your theist interlocutor to limit himself or herself to the first sentence of the letter, which does indeed go on to state in more explicit terms that Mosaic Law no longer applies in the author's view.

The "letter" (probably in fact written by Paul's buddy Priscilla) was a broadside delivered against what both he and she would have regarded as apostates but were in effect jewish christians who reckoned the descriptors were so synonymous in the new cult that nothing could be excised from the old laws (except male foreskins). Basically one had to be a jew to be a christian according to them.

Paul, Priscilla, Aquila and the gang on the other hand were famously on a different tack entirely, knowing full well that to have any chance of acquiring recruits from the gentile population (ie. the world, as they understood it and a potentially lucrative target group in terms of conversions) they would first have to get the "judaizers" to shut up. The letter, even though it's addressed to "Hebrews" is in fact targetting those jews in Rome calling themselves christian and messianic but ensconced very much in the traditional laws of their religion. Rome would have produced a variety of such hybrid views. The Pauline view was just another hybrid doing the rounds at the time, and may even have been in a noticeable minority amongst "christians" living in the metropolis.

So, while your theist is right to say that the letter repudiates Mosaic Law, it was still just a matter of opinion on the author's part, whoever he or she was. It would be a while yet before that issue was finally settled and, along with many other selectively promoted texts, before this letter would be retrogressively held up as a theological tenet suggesting a smooth and continuous departure of the cult from its jewish roots. No such thing happened.

Put simply, if the theist is attempting to prove in historical terms a clean break from Mosaic Law through reference to this letter then he or she has picked a bad example since the letter is, on the contrary, proof that the break was proving anything but clean at the time and that the issue was still up for grabs. Otherwise there was no need for it to have been written at all.

Sorry.  I know the laws of the old testament are ridiculous nonsense.  The point was that I was talking to someone who believes them, but that they no longer apply.  I was trying to look at the argument from his point of view.  It was most likely foolish of me to try to get any logic through his Jesus force field.

Agreed.

Often the most effective and adamant way to disabuse a religionist of their fallacial notions is to invalidate them within the context they purport to inhabit.

In this case the proponent of the fallacial argument is assuming an historical fact which is easy to debunk, and without even having to leave the terms of reference the theist herself proposed. That makes it all the sweeter.

Adopting the line "it's all garbage anyway" is essentially honest and true, but in an argument with a biased deluded interlocutor not worth much at all in terms of convincing them of the error of their ways. In fact it tends to lose the argument.