Going to an SSPX chapel doesn't necessarily mean the person has an iron grip on DOCTRINE, and that's the killer. Why do so many people loose their faith period? Often, it's because (or the excuse they have anyway is because) they had some question or problem with the faith, based on a misunderstanding or ignorance, that they didn't or couldn't clear up. Though the problem was with their understanding of the Faith and not the Faith itself, they nevertheless go flying out of the door (of the Church), firmly convinced it was all untrue.
Now realistically, most people quit the faith for agnosticism or atheism, and not really because they had an intellectual misunderstanding, but because they didn't want the burden of having to behave themselves, but... that's another story. Even if you take their word for it, though, the fact is when you present the truth to them in a way they can understand it, they often still just blow it off, because whether they want to be right, or whether they want to be "free" (of any moral restraint), the truth you can tell them is something they often simply do not want.
It's very difficult to reason with someone who has made up their minds against the truth. How do you explain truth to someone who has decided to hate it?
But it is also very difficult to continue to spend time with someone who becomes a danger to your own faith. If it comes to that, you cannot willingly endanger your faith. If the person continues to try to "debunk" your Faith, you may need to stay away from them. But that's up to you. I guess you should try to get the information to debunk their errors into their hands and heads, but... there comes a point when you're fighting in vain, and just drowning in their poison. (They're spewing garbage at you, while dismissing everything you say.)
Use prudence. Do what you can for the person, but if it comes to being a danger to your own faith, you have to protect your own faith first. We are not supposed to expose ourselves to unnecessary dangers on purpose, because we are only human, and since all of us are ignorant about SOME things, and subject to our own faults and flaws, all of us can be fooled or tricked at some point, if we just take in enough of the lies to twist our vision. All of us are capable of falling into error of some kind or other, under certain circumstances, simply because we are human and fallible.
The biggest mistake we can make is thinking "my faith is too strong to loose to protestantism". If you put yourself in the poison, you can't expect it won't effect you sooner or later. It's happened to Catholics before, and I'm sure it will keep happening, wherever we Catholics think too much of our own strength. Don't make that mistake!
"For our wrestling is not against flesh and blood; but against principalities and power, against the rulers of the world of this darkness, against the spirits of wickedness..."
If the person has a good will to know the truth, you can find information to refute what they say with the truth. But if they have their minds made up, and they are just blowing you off, and bent on them "being right", then you are probably just wasting your time, and just exposing yourself to unnecessary danger of hearing the slimy lies of protestantism repeated to you again and again.