Andrew, my post may also be too late to be of any help for you, but since no one else addressed the scripture verse that you are stuck on, I thought I would since I had a little time today.
Gal. 2:16 is making the distinction that a person is justified by ‘faith in Jesus Christ’ and not by ‘works of the Law’.
Those two observances just need to be defined a little.
‘Works of the Law’; -we know is referring to the works of the ‘Jewish’ Law, (verses 11-15). The ‘works’ are things like circumcision, sacrificing of animals, correct food to eat or not eat, how and when to wash.
As you know, the Catholic Church doesn't teach that this justifies a person. It also needs to be pointed out that there is also a distinction between ‘works’ of the Law, and keeping the commandments of God. Paul does it in 1Cor. 7:19.
‘Faith in Jesus Christ’; -that's what really needs to be defined in that bible verse (Gal.2:16).
What is it? What is 'real' faith in Jesus Christ? Is it anything less than trusting Jesus in EVERYTHING He taught and instructed to believe and do? The Catholic Church would say it is everything. What would that other person say? Jesus didn't say anything about keeping commandments as being optional, or repenting is optional, or being baptised is optional, or eating and drinking His body and blood is optional.. -and you could go on and on.
Also, when someone says they are ‘saved by faith alone’; they need to define exactly what that means, because from my experience in discussing with others it can mean just about anything.
Does ‘saved’ mean final salvation -as being perfect for heaven, or just justified or redeemed but not sanctified?
And then, what is ‘faith alone’? There are those who use that same phrase, but put completely different meanings to it. Is it just believing something is true, but not applying it to yourself or acting on what you believe? or is it acting on what you believe? Or do they really mean ‘grace’ and not faith?
One thing I learned if you are going to discuss religion and remain at peace, is not to let the argument be between you and the other person. Because it really is not. It's between the other person and 2000 years of authentic and historic Christianity. Let yourself be kind of a third person, who accepts the unchanged truths that have been handed down by an unbroken chain of apostolic succession under the guidence of the Holy Spirit.
I would also suggest you try to use the wording and statements that the church does regarding things, so it is not on your shoulders to correctly define christianity. The same questions have been answered over and over through the centuries.
In this area of concern read paragraphs 1987-2011 in the Catechism of the Catholic Church about Justification, Grace, and Merit.
Also, the JOINT DECLARATION ON THE DOCTRINE OF JUSTIFICATION from Vatican website.