tl;dr– Yes, your friend's theory is marginally falsifiable and therefore marginally "scientific", where "scientific" means "prone to scientific scrutiny". Of course, this doesn't mean that it's correct, nor is its falsifiable surface area particularly interesting.
My friend retorts: "Sure it is! My theory rests on the theory of gravitation, as gravity is what makes objects orbit Earth. If the theory of gravitation is falsified, then my theory is falsified. Thus my theory is falsifiable, and therefore scientific."
Yeah, sure, this is accurate-ish (ignoring the necessary-but-not-sufficient issue).
To be clear, when we say that theories must be falsifiable to be scientific, what we mean is that theories must be falsifiable to be prone to scientific analysis. Your friend's theory is marginally falsifiable, and so it's marginally prone to scientific scrutiny. For example, as your friend correctly notes, their theory could be falsified if gravity is disproven.
Of course, a theory being scientific (i.e., prone to scientific scrutiny) doesn't make it correct.
Nor does a theory being scientific (in the sense of being prone to scientific scrutiny) make it scientific (in the sense of being verified by scientific analysis).
Also, we're currently looking for such invisible-but-gravitational unicorns.
Note: "Scientific" means something different from normal here.
Folks probably think of "scientific" theories as those supported by scientific research.
In the context of falsifiability, "scientific" means "able to be scrutinized by scientific research".
For example, say Alice asserts that Luna is, in fact, made of cheese – a fine white cheddar, to be precise. Then:
Alice's theory is "scientific" in the sense of it being prone to scientific analysis.
Alice's theory is not "scientific" in the sense of being supported by scientific analysis.
Note: There's a framing ambiguity complicating the situation.
Consider a student handing in their homework:
Student: Here's my research paper!
[The teacher glances over the paper.]
Teacher: This was supposed to be a referenced research paper, but I don't see a single citation!
Student: Check out the cover page! I cited my birth certificate, state ID, passport, and social-security card to reference my name, giving me 4 authoritative sources!
Is the student's paper "referenced"?
Yes, it is! The student did include references, so the overall paper is, in fact, referenced.
That said, the teacher's likely to be unimpressed as the significant bulk of the paper isn't referenced, despite the totality of it being referenced.
Likewise, your friend's theory about invisible pink unicorns has a significant bulk of it that may be unfalsifiable. So while your friend may correctly point out that the totality is falsifiable, this isn't a defense against the observation that a good bulk of it isn't.
Note: Falsifiability is a very low bar.
Unfalsifiable theories have also been called "not even wrong".
We say that they're "not even" wrong because being unfalsifiable is worse than merely being wrong. Wrong theories were at least meaningful enough to get to the point where they could be called wrong; by contrast, not-even-wrong theories fail to even get that far.