First of all, I think it is clear that it has to do with the different understanding of salvation that your friend or his/her church community has. The Catholic Church believes that while baptism and faith that is practised as taught by the Church are the best and the safest way that leads to salvation, we also believe that God is not limited by it, especially when the person, not being at his own fault, did not have a chance of making that decision. God prefers baptism and faith, but God also makes sure in his own mysterious ways that everyone has a fair chance.
This belief, however, is not shared by a lot of the Protestant brothers and sisters, so while you can share this view with them, it might not be very useful.
Therefore, I would encourage this person to check with his/her pastor. I am pretty sure most if not all denominations would disagree with his/her line of thinking, and maybe his/her own pastor can best assist your friend, since they share the same beliefs.
Meanwhile, even your friend acknowledged that it is only a possibility that his/her parents were damned. I think that is an important point. That is also saying that he/she is still open to the possibility that his/her parents are in fact not in hell, but in heaven or purgatory.
Of course, they don’t understand purgatory, but it is here that it may be the most favourable opportunity for you to share about this Catholic belief. You see, the teaching on purgatory is actually an expression of God’s mercy. Yes, it is true only the perfect can be standing face to face before God in heaven; yet only those who definitively reject God deserve hell. God is merciful, and so He allows us time after death to be purified, as Jesus alluded to in some of the parables that talk about needing to pay off the debt over a long time (see Matt 18:21ff). If there is time to pay, that means there is an end to the suffering. And so there is a chance of purgation that does not lead to hell, but to heaven. That is indeed very merciful, as Jesus always taught.
And if there is a chance the parents are on the way to heaven, even for such a chance, we must not entertain such a thought as wanting to go to hell. Hell is not a place to desire to go. And besides, your friend’s purpose to go there is to be with his/her parents, which is out of love. But that’s precisely not found or allowed in hell. One cannot be in hell for love, nor can one exercise love in hell. There is no compassion there possible, but only hatred and agony left. Going there is out of solidarity, but it will not be honoured. It will not bring any good to anyone, for once a soul goes there, either love will keep him out, or hatred will consume him, for the very nature of hell is the absence of all good.