I think Tessa's explanation shows that they will make it. She does understand the process of the show and how really, it's just fate (or dumb luck) that they've found each other.
Honestly, she should probably never have watched any of the show. None of us get a bird's eye view of our boyfriend/girlfriend/husband/wife past relationships. Just because it took place at the same time as the beginning of her relationship with Andy doesn't mean anything about where "Tandy" will end up. I hope they can view this as a past and not as something to pick and poke at.
Good luck and happiness to them!